The High-Conductivity Materials Race: Diamond, θ-TaN, SiC and AlN

For over a century, thermal design in electronics had one anchor: copper, at roughly 400 W·m⁻¹·K⁻¹ — a value nailed down by decades of standards-laboratory metrology¹ and treated as the practical ceiling for metallic heat spreading. In the span of a few years that comfortable ordering has been upended. A metallic nitride now measures near three times copper. Synthetic diamond single crystals sit near five times. Composites of diamond and copper have been reported in deployed supercomputing hardware. Wide-bandgap ceramics that were once secondary to copper in thermal-management discussions are now load-bearing in advanced packages. This article ranks the contenders — and, because a number without its measurement conditions is a rumor, it attaches the condition tag to every rung.

The copper century, and why it ended

Copper earned its position honestly. Its conductivity is high, stable, isotropic, and — critically — known: the standards-laboratory compilations that anchor its ~400 W·m⁻¹·K⁻¹ figure span temperature and purity with documented provenance¹. Aluminum, its cheaper understudy, runs substantially lower and swings widely with alloy and temper. For decades that pairing was enough, because fluxes were modest and the spreader was rarely the bottleneck. What ended the copper century was not a better metal appearing — it was the flux problem described in our thermal-wall analysis outgrowing what any ~400-class spreader can flatten. Once hotspot flux climbs into the tens of watts per square centimeter and beyond, spreading resistance can become a first-order term, depending on source-to-spreader geometry and the rest of the stack, and the search for higher-conductivity materials stops being academic.

One caution before the leaderboard: in metals, heat rides mostly on electrons, and conventional metallic thermal transport is constrained by several intrinsic scattering mechanisms, including electron–phonon coupling — and the θ-TaN result is unusual precisely because it combines exceptionally weak electron–phonon coupling with favorable phonon transport. In crystals like diamond, heat rides on phonons, whose ceiling is set by different physics. Comparing entries across that divide is legitimate — heat is heat — but the form of the material matters enormously, which is where the condition tags come in.

A second, quieter reason copper anchored the era: at bulk scale its number is transferable. Rolled lids, wrought cold plates and machined blocks land within a predictable band of the reference value. That forgiveness has limits, though — electroplated vias, nanoscale films, fine-grained interconnect copper and contaminated deposits can fall well below bulk because grain-boundary, impurity and surface scattering tax the electrons. The challengers below enjoy even less forgiveness — their headline values are single-crystal properties, and every step toward a manufacturable form taxes them. Keep that asymmetry in mind as the ladder climbs.

θ-TaN: a metal that conducts like a crystal

The 2026 result that rearranged the metallic leaderboard is single-crystalline θ-phase tantalum nitride, reported at approximately 1,100 W·m⁻¹·K⁻¹ at room temperature — close to three times copper, and a reported record among metallic materials². The mechanism is as interesting as the number: synchrotron inelastic X-ray scattering revealed a phonon band structure with a large acoustic–optical gap and phonon bunching that suppress phonon–phonon scattering, while ultrafast spectroscopy confirmed exceptionally weak electron–phonon coupling — the century-old ceiling was not broken so much as sidestepped. Two condition tags matter. First, the division of labor among the techniques matters: TDTR measured the thermal conductivity, while the inelastic X-ray scattering and ultrafast spectroscopy supported the proposed transport mechanism — and the value belongs to that specimen form. Second, TaN is familiar in interconnect technology as a diffusion-barrier material, which makes the integration story tantalizing — but the record belongs to metastable single-crystal θ-TaN, not to conventional sputtered barrier films, and process compatibility is a materials-integration question that must be demonstrated rather than assumed.

Diamond: the ceiling above the ceiling

High-purity synthetic single-crystal diamond measures around 2,000 W·m⁻¹·K⁻¹ at room temperature³ — phonon transport in one of the stiffest, lightest lattices in nature. It also brings a low coefficient of thermal expansion, which is why so much engineering effort goes into diamond/copper composites that trade some conductivity for machinability and CTE matching; deployed composite material has been reported above 1,000 W·m⁻¹·K⁻¹ in a megawatt-class immersion-cooled cabinet⁴. The condition tags: the ~2,000 figure belongs to high-purity single crystals; CVD polycrystalline plates run lower and vary with grain structure; and a composite’s number is a property of that composite system — filler loading, interface engineering, consolidation — reported per system, not per ingredient. The engineering realities of getting diamond’s conductivity to actually flatten a hotspot are the subject of the companion spreader article.

SiC and AlN: the packaging workhorses

Below the headline rungs sit two ceramics doing quieter, load-bearing work. Silicon carbide’s best thermal showing is crystal-form specific: high-purity, wafer-scale free-standing cubic (3C) SiC crystals have demonstrated isotropic room-temperature conductivity exceeding 500 W·m⁻¹·K⁻¹⁵ — while other polytypes, doped wafers, films and sintered ceramics span a wide lower range. SiC is an established wide-bandgap device and substrate material, with potential relevance to thermally demanding heterogeneous integration; it should not be described as a standard interposer material in mainstream 2.5D flows, which use silicon interposers. Aluminum nitride is the more instructive case for a metrology-minded reader. Its intrinsic single-crystal conductivity, approximately 320 W·m⁻¹·K⁻¹, was established on ultra-pure crystals decades ago⁶ — but commercial AlN ceramics commonly report values well below the intrinsic single-crystal limit, with performance strongly dependent on oxygen content, additives, porosity and grain structure. A 2020 study experimentally confirmed the intrinsic value on MOCVD-grown material pure enough to approach the theoretical limit⁷. And in thin-film form the story inverts again: micrometer-thick AlN films have been measured with in-plane conductivity around 260 W·m⁻¹·K⁻¹ — among the highest for any film of that thickness, yet far from the bulk intrinsic number⁸. One chemical formula, three different materials, three different measurements. That is the leaderboard’s fine print in miniature.

Graphene: the anisotropic wildcard

Suspended high-quality monolayer graphene delivered some of the highest thermal conductivities ever reported — values near 4,800–5,300 W·m⁻¹·K⁻¹ in the landmark opto-thermal Raman study, with the absorbed-power calibration dominating the error bar⁹; supported, polycrystalline and assembled graphene materials can be far lower. But graphene’s defining thermal property is anisotropy: superb in-plane, poor through-plane. Macroscopic graphene assemblies inherit this — graphitized films and fibers can carry heat brilliantly along their alignment direction¹⁰, while through-thickness transport lags by orders of magnitude, a contrast quantified directly by measuring in-plane and out-of-plane conductivity on the same graphene-based films — the intrinsic anisotropy-ratio study from this laboratory’s research lineage¹¹. That makes graphene a genuine contender as a lateral heat-spreading layer and a poor default as a through-stack conductor — suitability is a direction, not a scalar. Our graphene series covers the material forms this applies to.

One scope note: this is an application-focused shortlist for chip-level heat spreading, not an exhaustive ranking of every high-conductivity crystal — cubic boron arsenide, isotope-engineered cubic BN and others also compete at the top of the phonon leaderboard.

The film problem: why the hardest numbers to verify matter most

Notice where the race is actually being run: not in billets but in films, barriers, interposers and spreading layers — forms micrometers thick or less. Thin films are precisely where thermal metrology is least forgiving. A film on a substrate leaks heat into its support, so the measurement must separate film from substrate signal; in-plane and through-plane transport can diverge substantially, so a single scalar is often insufficient for an anisotropic application; and at micrometer thicknesses, boundary scattering makes conductivity a function of the very dimension being measured — the AlN films above are high for films while sitting far below their own bulk crystal⁸. Techniques exist for exactly this regime — suspended-specimen transient methods free a film from its substrate entirely and read its in-plane transport directly¹², while pump–probe thermoreflectance interrogates supported films from above — but they must be chosen, and their answers must be labeled with direction and form. A leaderboard that ignores the film problem is a list of materials nobody can buy.

The ladder, with condition tags

The interactive below ranks the contenders by their headline room-temperature values — and refuses to show a number without its provenance. Click any rung.

Read across the tags and a pattern emerges: the highest rungs in this shortlist often come with more restrictive specimen conditions. Alloys at the bottom vary with processing; the highest published values often belong to exceptional specimen forms — single crystals or suspended monolayers — measured with specialized methods under tightly controlled conditions. The ladder is real. The ladder is also a ranking of measurements.

Reading the leaderboard like a metrologist

Three habits turn the leaderboard from marketing into engineering. First, match the form: single crystal, film, ceramic, composite and fiber are different materials sharing a name; a value transfers only within its form. Second, match the direction: for anisotropic SiC polytypes, textured AlN films, graphene and other layered or aligned materials, in-plane and through-plane transport can differ by anything from tens of percent to orders of magnitude — ask which one the datasheet reports. Third, match the method: TDTR provides a near-surface, depth-weighted response whose sensitivity depends on modulation frequency, transducer properties and the full multilayer stack; flash methods average a whole disc; suspended-specimen transient methods read a fiber’s axis¹². None is wrong; each answers a different question, and the wrong match imports someone else’s answer into your design. When a supplier’s number and your application’s question do not line up — or when the lot in your hand needs its own number — that is a measurement task, and the pillar guide maps which method answers which question. The race is real, and it will be won by whoever measures honestly enough to iterate faster.

Keep Exploring the ACS Thermal Metrology Knowledge Hub

This article is one chapter of the ACS thermal metrology knowledge hub. To keep going:

• Thermal conductivity & diffusivity testing: the pillar guide — methods, samples and a buyer’s framework in one place.
• Why AI chips hit the thermal wall — the heat-flux problem behind the packaging race.
• Diamond and diamond–copper spreaders — from lab curiosity to chip-level hardware.
• Graphene for thermal management — heat-spreader films, fibers and composites.
• Graphene Series materials — the material family these articles measure.