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A team led by Prof. Bivas Saha at JNCASR, Bengaluru, working with the University of Sydney and IISc Bengaluru, has reported a thermoelectric voltage in a crystalline solid nearly a thousand times larger than the accepted ceiling for such materials.
The material is scandium nitride (ScN), grown as epitaxial thin films on magnesium oxide substrates by ultrahigh-vacuum magnetron sputtering and deliberately doped with magnesium to produce a heavily doped, highly compensated (HDHC) semiconductor.
The measured Seebeck coefficient exceeded -124.6 millivolts per kelvin near room temperature in a roughly 200-nanometre-thick film - a range previously seen only in liquid electrolytes and ionic gels, never in single-crystalline solids.
A prototype photon sensor made from an HDHC ScN film with two chromium contacts gave a Seebeck response of -102.4 millivolts per kelvin when one contact was illuminated by a laser; an Indian patent application has been filed.
The work, carried out at JNCASR (an autonomous institute of the Department of Science and Technology), was published in Science and points to ultrasensitive temperature sensors, thermal imaging, bolometers, IoT sensors and cryogenic single-photon detectors for quantum technologies.
When one end of a junction between two dissimilar materials is heated and the other kept cold, mobile charge carriers drift from the hot side to the cold side and pile up, building a voltage across the junction. This is the Seebeck effect, discovered by Thomas Johann Seebeck in 1821. The size of the voltage per degree of temperature difference is the Seebeck coefficient (S), and it is what thermocouples, thermoelectric generators and the radioisotope thermoelectric generators on deep-space probes all depend on. Its converse is the Peltier effect, reported by Jean Charles Athanase Peltier in 1834: pass a current through a junction of two dissimilar conductors and heat is absorbed or released there depending on the current's direction - the basis of solid-state Peltier coolers. A material's overall thermoelectric usefulness is measured by the dimensionless figure of merit ZT = S-squared x sigma x T / kappa, defined by A.F. Ioffe in 1957, where sigma is electrical conductivity, T the absolute temperature and kappa the thermal conductivity. A good thermoelectric therefore needs a large Seebeck coefficient AND high electrical conductivity AND low thermal conductivity - three properties that ordinarily fight each other, which is why practical thermoelectric efficiency has improved so slowly.
Simple Analogy: Heat pushes charge carriers down a material the way wind pushes leaves down a street: they pile up at the cold end, and that pile-up is the voltage. Until now, in a solid crystal the pile could only get so high - this result shows the fence was in the wrong place.
| Material class | Typical Seebeck coefficient | Charge carrier |
|---|---|---|
| Ordinary metals | Tens of microvolts per kelvin | Electrons |
| Good inorganic semiconductors | Rarely above a few hundred microvolts per kelvin | Electrons and holes |
| Accepted ceiling for a crystalline solid | A few millivolts per kelvin | Electrons and holes |
| Ionic gels, hydrogels and liquid electrolytes | Millivolts per kelvin range | Ions |
| HDHC scandium nitride film (this work) | Exceeding -124.6 millivolts per kelvin near room temperature | Electrons |
Autonomous institute of the Department of Science and Technology and a deemed university; set up in 1989 to mark the birth centenary of Jawaharlal Nehru, founded on the initiative of Bharat Ratna Prof. C.N.R. Rao. Led this study.
Established in May 1971 under the Ministry of Science and Technology; the nodal department for organising, coordinating and promoting science and technology activities, and parent of a large family of autonomous research institutes including JNCASR.
India's premier institute for advanced scientific and technological research, established by a vesting order of 27 May 1909 with support from Jamsetji Tata and land from the Mysore Durbar. Prof. Subroto Mukerjee of IISc was part of the team.
Australian partner institution; Ashalatha Indiradevi Kamalasanan Pillai and Dr. Magnus Garbrecht contributed the microscopy and characterisation work.
The Seebeck effect is what powers deep-space probes: heat from decaying plutonium-238 is converted directly to electricity with no moving parts. A recurring theme whenever nuclear batteries or planetary missions are in the news.
Thermoelectric generators are one route to capturing industrial waste heat, linking this to the Bureau of Energy Efficiency's mandate, the PAT (Perform, Achieve and Trade) scheme and India's Net Zero 2070 commitment.
Single-photon detectors are core hardware for quantum communication and quantum sensing - two of the verticals of the National Quantum Mission - so a room-temperature route to them has direct strategic value.
The everyday application of the Seebeck effect; a thermocouple in an industrial furnace is the same physics, at microvolts per kelvin instead of millivolts.
The converse effect, used in portable coolers, CPU spot-cooling and laboratory instruments - the standard follow-up once the Seebeck effect appears in a paper.
GS Paper 3 > Science and Technology > Developments and their applications; indigenous technology
General Science (Physics) > Heat and electricity; Current Affairs > Science and research institutions
General Science > Physics > Thermoelectric effects
Thermoelectric effects are a standing General Science topic in SSC and Railway papers; institution-to-parent-department pairings appear across all exams.
The thermoelectric voltage generated per unit temperature difference across a material, usually quoted in microvolts or millivolts per kelvin.
Dimensionless measure of thermoelectric performance, ZT = S-squared x sigma x T / kappa; defined by A.F. Ioffe in 1957.
A film grown so that its crystal lattice lines up with that of the underlying substrate, producing a single-crystalline layer rather than a polycrystalline one.
A semiconductor carrying large and nearly equal numbers of randomly distributed donor and acceptor dopants, so that the two largely cancel each other's free carriers.
A material that retains its strength and structure at very high temperatures; ScN, with a melting point around 2600 degrees Celsius, qualifies.
A device that measures incident radiation by detecting the tiny temperature rise it causes.