A Breakthrough in Thermoelectricity
Scientists have discovered a way to generate an unusually large electrical voltage from a small temperature difference, challenging a limit that has guided the understanding of solid materials for over a century. The breakthrough involves a specially engineered semiconductor made from scandium nitride (ScN). Researchers from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), the University of Sydney and the Indian Institute of Science (IISc) found that the material can produce a thermoelectric voltage far beyond what was previously considered possible in crystalline solids. The findings were published in the journal Science.
The discovery is rooted in the Seebeck effect, a phenomenon discovered two centuries ago. In simple terms, when one side of a material is hotter than the other, mobile charge carriers drift from the hot side to the cold side, building up a voltage across the material. This effect is already used in devices such as temperature sensors and systems that convert waste heat into electricity.
For decades, scientists believed there was a practical upper limit on how large this voltage could become in a crystalline solid. Most metals generate just tens of microvolts per Kelvin, and even good semiconductors rarely exceed a few hundred microvolts per Kelvin. Only liquid systems, such as ionic gels and electrolytes, in which charged ions rather than electrons carry the heat, had been known to cross into the millivolts-per-Kelvin range. The new research challenges that assumption.
The Science Behind the Discovery
The team, led by Prof Bivas Saha, grew thin films of scandium nitride on magnesium oxide substrates using ultrahigh-vacuum magnetron sputtering. They deliberately introduced magnesium into the material to compensate for naturally occurring free electrons from oxygen dopants. This produced what researchers call a heavily doped, highly compensated semiconductor, altering how charge moves through the crystal.
In one of these films, researchers measured a Seebeck coefficient exceeding -124.6 millivolts per Kelvin near room temperature. According to India Today, this is nearly 100 times beyond the earlier reported ceiling for crystalline solids. The Times of India reported that the response was several hundred to more than a thousand times larger than typically seen in inorganic semiconductors, and The Hindu noted that the voltage rivals values normally seen only in liquid electrolytes and ionic gels.
X-ray diffraction and atomic-resolution electron microscopy confirmed that the films remained single-crystalline and epitaxial, with dopant atoms distributed uniformly and no secondary phases or precipitates, as reported by The Hindu. The effect became even stronger as the films were made thinner, according to India Today.
Potential Applications
The discovery could lead to much more sensitive devices for detecting heat and tiny temperature changes, including applications in thermal imaging and quantum technologies. The researchers have already built a preliminary photon sensor using the material. When a laser illuminated one of its contacts, it generated a Seebeck response of -102.4 millivolts per Kelvin, as reported by India Today.
The Department of Science and Technology said the discovery "overturns a decades-old assumption about the ceiling on this effect in solids and opens a path to a new generation of ultrasensitive temperature sensors, heat detectors, and quantum sensing devices."
An Indian patent application has been filed covering thermoelectric thin-film materials and sensors based on the work, as reported by both India Today and The Times of India.