Start with the number that makes this worth reading. A modern thermal power station converts only about a third of the energy in its fuel into electricity. Most of the rest leaves as heat — up flues, through cooling towers, into rivers and into the air. The same is true of a cement kiln, a steel plant, a car engine and, increasingly, a data centre. Waste heat is the single largest stream of discarded energy in the industrial world.
A device that turned even a small part of that stream back into electricity, with no moving parts, would be valuable. Such devices exist. They are called thermoelectric generators, and the reason they are not everywhere is that the materials inside them are not good enough. On 14 September 2026 the Ministry of Science and Technology announced work from JNCASR, Bengaluru that improves the materials science underneath — and does so by finding heat behaving in a way that solids are not supposed to allow.
The effect the device runs on
The physics is nearly two centuries old. Hold the two ends of a suitable material at different temperatures and a voltage appears across it. This is the Seebeck effect.
The mechanism is intuitive. Charge carriers at the hot end have more thermal energy and move more vigorously. They diffuse towards the cold end, and as they accumulate there, charge separates: one end becomes more negative, the other more positive. That separation is a potential difference, and it will drive a current through an external circuit. Connect many such elements and you have a generator that runs on a temperature difference alone.
Its mirror image is the Peltier effect: pass a current through a junction of two different conductors and heat is absorbed at one junction and released at the other. That is how portable solid-state coolers and some electronics-cooling modules work. Seebeck turns heat into electricity; Peltier turns electricity into a heat flow. There is a third, the Thomson effect, describing heating or cooling along a single conductor carrying current in a temperature gradient. All three belong with the thermodynamics and current electricity sections of the syllabus.
Thermoelectric generators have one great virtue: no moving parts. Nothing spins, nothing wears, nothing needs lubrication. That is why they have flown on deep-space missions for decades, where a radioisotope supplies the hot end and the cold of space supplies the other, and where a mechanical generator would be an unacceptable risk. The limitation is equally plain — efficiency.
Why good thermoelectrics are hard to find
Performance is measured by a dimensionless quantity, the thermoelectric figure of merit:
zT = S²σT / κ
where S is the Seebeck coefficient (volts generated per degree of temperature difference), σ is electrical conductivity, T is absolute temperature, and κ is total thermal conductivity.
Read the fraction and the engineering problem states itself. To raise zT you want S large, σ large, and κ small. In other words you need a material that conducts electricity well but conducts heat badly — a combination sometimes called a "phonon glass, electron crystal".
Nature resists. In ordinary metals the same free electrons that carry charge also carry heat, so σ and κ rise and fall together; that relationship is close to the Wiedemann–Franz law. Push one up and the other follows. Decades of thermoelectric research have therefore concentrated on the one part of κ that can be attacked separately.
Total thermal conductivity has two components:
- Electronic thermal conductivity, carried by the charge carriers — tied to σ, and hard to suppress without ruining electrical performance.
- Lattice thermal conductivity, carried by vibrations of the atoms themselves.
The lattice part is the target. Suppress vibrational heat transport without touching the electrons and zT rises.
Phonons, and what "wave-like" means here
Atoms in a crystal are not stationary; they vibrate about fixed positions, and because they are bonded to their neighbours, a vibration propagates. Quantised, these collective vibrations are called phonons, and in an ordinary crystalline solid phonons are the principal carriers of heat.
The standard picture treats a phonon as a particle that travels a certain distance before scattering — its mean free path — which in a good crystal is much longer than the spacing between atoms. Heat then flows as a gas of these particles diffusing through the lattice. In a glass, by contrast, disorder scatters vibrations so severely that the mean free path shrinks to about the interatomic distance, and conventional phonon propagation essentially stops. Glasses conduct heat poorly for precisely this reason.
The JNCASR result concerns the territory in between, and this is the conceptually interesting part. A material can keep long-range crystalline order — it is a proper crystal, with a repeating lattice — while its vibrational modes become highly localised and closely spaced in energy, interacting strongly with one another. In that regime heat no longer moves only as particle-like phonons hopping along. It also moves by wave-like coherence between different vibrational modes, with energy tunnelling between neighbouring localised states rather than being carried by well-defined travelling particles.
Because that second channel is intrinsically inefficient, the material ends up with ultralow lattice thermal conductivity while remaining a crystal — glass-like heat transport without glass-like electrical properties. That is the "phonon glass, electron crystal" ideal, arrived at from an unexpected direction.
The material
The compound is thallium copper selenide, TlCu₅Se₃ — a copper chalcogenide. Chalcogenides are compounds of the group-16 elements (sulphur, selenium, tellurium) and have long been the most productive family in thermoelectrics.
Copper chalcogenides were already known for low thermal conductivity, but through a different and troublesome mechanism. Many are superionic: the copper ions become so mobile that they diffuse through the crystal almost like a liquid within a solid framework. That "liquid-like" motion scatters heat very effectively — but ions that migrate freely also migrate under an applied electric field, which causes the material to change composition and degrade in service. Excellent laboratory numbers, poor device lifetimes.
The JNCASR insight was to look for confined ion dynamics instead: copper atoms that are dynamically disordered but locked inside a complex framework so they cannot migrate over long distances. The structural reason this compound provides that is distinctive — TlCu₅Se₃ crystallises in a tetragonal structure forming a complex three-dimensional cloverleaf knot-like framework with open channels along the crystallographic c-axis. The bonding hierarchy of that framework restrains the copper atoms while still allowing them to rattle.
The result is exceptionally strong anharmonicity. A perfectly harmonic vibration is symmetric, like an ideal spring; anharmonicity means the atomic vibration is asymmetric and departs from that ideal, which makes phonons interact strongly with one another and disrupts orderly heat propagation. Strong anharmonicity plus confined disorder produced predominantly wave-like transport and ultralow lattice thermal conductivity — without the structural instability of the superionic route.
Combined with favourable electronic properties, the material reached a zT of about 1.7, among the highest reported for a pristine ternary chalcogenide. For scale, commercial thermoelectric materials have historically sat near zT ≈ 1, so this is a substantial figure.
Who did it, and where it might go
The work was led by Prof. Kanishka Biswas with Sayantoni Choudhury and Dr. Animesh Bhui of the New Chemistry Unit at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, an autonomous institution under the Department of Science & Technology. The theoretical side — first-principles calculations and molecular-dynamics simulations, which showed that the copper atoms exhibit localised dynamic disorder rather than long-range liquid-like diffusion — was done with Prof. Umesh V. Waghmare and Dr. Prasad V. Matukumilli of the Theoretical Sciences Unit at the same institution. The findings were published in Science Advances.
To analyse transport the team went beyond the conventional phonon-gas model and used the unified formalism of thermal transport, which accounts for both particle-like propagation and wave-like coherence between phonon branches — a reminder that a new experimental regime often needs a new theoretical frame to be seen at all.
The applications named in the announcement are the obvious industrial heat sources: power plants, cement industries, steel plants, automobiles, data centres and battery thermal management. Ultralow-thermal-conductivity materials matter beyond power generation too — in thermal barrier coatings for turbine blades, and in quantum technologies, where stray vibrational heat is a source of decoherence.
Two cautions belong in any honest answer. Thallium is highly toxic, which constrains mass deployment of this specific compound. And laboratory zT does not translate directly into device efficiency, which depends on module design, contact resistance and the temperature difference actually available. The value of the work is the mechanism it establishes — that structural complexity and confined ion dynamics can be used deliberately to engineer heat transport — more than this one compound.
🔑 Revision block
The finding. Researchers at JNCASR, Bengaluru (autonomous under DST) reported an unusual wave-like thermal transport regime in thallium copper selenide, TlCu₅Se₃, achieving a thermoelectric figure of merit zT of about 1.7 — among the highest for a pristine ternary chalcogenide. Published in Science Advances.
The people. Prof. Kanishka Biswas, with Sayantoni Choudhury and Dr. Animesh Bhui (New Chemistry Unit); theory with Prof. Umesh V. Waghmare and Dr. Prasad V. Matukumilli (Theoretical Sciences Unit).
The effect. Seebeck effect — a temperature difference produces a voltage. Reverse: Peltier effect — a current produces a heat flow. Also Thomson effect along a single current-carrying conductor in a temperature gradient.
The formula. zT = S²σT/κ, with S the Seebeck coefficient, σ electrical conductivity, T absolute temperature, κ total thermal conductivity. Want S and σ high, κ low.
Why it is hard. In metals, electrons carry both charge and heat, so σ and κ rise together (Wiedemann–Franz). Only the lattice part of κ can be attacked independently.
Phonons. Quantised lattice vibrations, the main heat carriers in a crystal. Crystal: mean free path ≫ interatomic distance, particle-like transport. Glass: mean free path ≈ interatomic distance, propagation suppressed. This material: crystalline order retained, but localised closely spaced modes give wave-like coherence and ultralow lattice thermal conductivity.
The structural trick. Confined copper dynamics inside a tetragonal, cloverleaf knot-like 3D framework with open channels along the c-axis — avoiding the instability of superionic conductors where ions migrate freely. Produces strong anharmonicity (asymmetric, non-ideal vibration).
Uses. Waste-heat recovery in power plants, cement, steel, automobiles, data centres, battery thermal management; thermal barrier coatings; quantum technologies. Thermoelectric generators have no moving parts — hence their use on deep-space missions.
🎯 Practice MCQs
Q1. The generation of a voltage across a material whose ends are at different temperatures is called the: (a) Peltier effect (b) Seebeck effect (c) Thomson effect (d) Hall effect → (b) — the Peltier effect is its converse.
Q2. In the expression zT = S²σT/κ, a higher zT is favoured by: (a) Lower electrical conductivity (b) Lower thermal conductivity (c) Lower Seebeck coefficient (d) Lower absolute temperature → (b) — κ sits in the denominator.
Q3. Phonons are: (a) Charge carriers in semiconductors (b) Quantised lattice vibrations (c) Photons of infrared frequency (d) Magnetic excitations → (b) — the principal heat carriers in a crystalline solid.
Q4. The Wiedemann–Franz law relates a metal's thermal conductivity to its: (a) Density (b) Specific heat (c) Electrical conductivity (d) Melting point → (c) — which is why the electronic part of κ is hard to suppress on its own.
Q5. TlCu₅Se₃ belongs to which family of compounds? (a) Oxides (b) Nitrides (c) Chalcogenides (d) Halides → (c) — compounds of group-16 elements; here, selenium.
Q6. The main practical drawback of superionic thermoelectric materials is: (a) Very high thermal conductivity (b) Mobile ions migrate under an applied field, causing structural and compositional instability (c) They cannot be synthesised above room temperature (d) They have zero Seebeck coefficient → (b) — which is what confined ion dynamics is designed to avoid.
Q7. JNCASR, where the work was carried out, is an autonomous institution under the: (a) Ministry of Education (b) Department of Science & Technology (c) CSIR (d) Department of Atomic Energy → (b) — located in Bengaluru.
Q8. Anharmonicity in lattice vibrations means that: (a) Atoms do not vibrate at all (b) Vibrations are asymmetric and deviate from ideal harmonic behaviour (c) All phonons have the same frequency (d) The crystal has lost long-range order → (b) — strong anharmonicity makes phonons interact and disrupts heat propagation.
Q9. A thermoelectric generator is preferred for deep-space missions mainly because it: (a) Is the lightest possible power source (b) Has no moving parts and so is highly reliable (c) Works only in vacuum (d) Requires no temperature difference → (b) — reliability over a mission lasting years.
Q10. Consider the following statements: 1. In a glass, the phonon mean free path is reduced to nearly the interatomic distance. 2. TlCu₅Se₃ achieves low lattice thermal conductivity by losing its long-range crystalline order. (a) 1 only (b) 2 only (c) Both (d) Neither → (a) — the material retains crystalline order; that is precisely what makes the result unusual.
📋 How this gets asked (PYQ pattern)
Science-and-technology items in the NDA General Ability Test rarely ask for the research result itself. They ask for the physics the result sits on, which is why this announcement is worth studying rather than merely noting. Expect four shapes. The effect-naming item gives a description and asks whether it is Seebeck, Peltier, Thomson or Hall — the Seebeck/Peltier direction is the classic swap. The carrier item asks what transports heat in a solid, with phonons against photons, electrons and protons as options. The institution item asks which department or ministry an institute sits under; DST, CSIR, DAE and DBT are rotated, and JNCASR under DST is the pairing here. The application item asks where a technology is used, with deep-space power supply the standard correct option for thermoelectrics.
A fifth shape is becoming common as papers modernise: the two-statement item combining a definition and a consequence — for instance, that a good thermoelectric must conduct electricity well and heat poorly. That formulation tests whether a candidate has understood zT rather than memorised it.
The fresh 2026 hooks are TlCu₅Se₃, the zT of about 1.7, JNCASR under DST, and the phrase wave-like heat transport. Also worth carrying is the journal, Science Advances, since named-journal questions do appear. We describe the recurring pattern here, not any exact past question.
Preparing for the NDA? Physics in the GAT rewards students who can explain a mechanism, not just name it — one properly understood effect answers several differently worded questions. Build the base with our NDA physics hub, follow the daily NDA current affairs, and prepare with our faculty in the upcoming Cavalier courses in Delhi.
✍️ Written by The Cavalier Faculty Desk — Science & current affairs team at The Cavalier. The Cavalier, founded by ex-Army officers, has trained NDA/CDS/SSB aspirants since 2001 (Facebook · YouTube).
Source: PIB / Ministry of Science & Technology, 14 September 2026 (Release ID 2310168). The zT definition, the Wiedemann–Franz relationship, phonon transport regimes and the reported zT value cross-verified with the published literature on thermoelectric materials and the Science Advances paper.