Annealed pyrolytic graphite
Synthetic graphite with exceptional in-plane heat spreading ability.
Annealed Pyrolytic Graphite (APG), also called Thermally Annealed Pyrolytic Graphite (TPG), is a synthetic graphite engineered for exceptional heat conduction within its plane. Like pyrolytic carbon and pyrolytic graphite (PG), it is lightweight, electrically conductive, and diamagnetic—meaning it can levitate in a magnetic field.
APG is anisotropic: its layered structure gives it very high in-plane thermal conductivity—1,700 W/m·K at room temperature—while through its thickness, conductivity is low. This laminate structure stays stable over a broad temperature range, so the material works well for moving heat. Its in-plane conductivity actually rises as temperature drops, peaking at 2,800 W/m·K around 150 K. Unlike standard pyrolytic graphite, APG’s conductivity is uniform across every basal plane, from the center to the outer layers. The high in-plane conductivity and stiffness come from covalently bonded carbon atoms arranged in a hexagonal pattern. Between these planes, weak van der Waals bonds result in poor through-thickness thermal conductivity, stiffness, and strength.
APG is made similarly to Highly Oriented Pyrolytic Graphite (HOPG). Hydrocarbon gas is heated until it breaks down into carbon, and pyrolytic graphite is grown on plates via chemical vapor deposition (CVD). This PG is then annealed at high temperature to create the more planar, uniform carbon structure of APG. The key difference from HOPG is that APG’s annealing does not require induced stresses, making it more affordable and practical for bulk production.
APG is mainly used as a heat spreader for thermal management in high-end electronics. Because its mechanical properties are poor, it is usually encapsulated in a structural metal—most often aluminum, chosen for its strength, low weight, cost, ease of manufacturing, and thermal conductivity. Since APG conducts heat poorly through its thickness, thermal vias (typically aluminum or copper) are sometimes inserted to transfer heat into the graphite. Thin, flexible APG sheets can be encapsulated in flexible materials like polymers, aluminum foil, or copper foil to form a thermal strap.
In aerospace, aluminum-APG plates serve as heat spreaders to draw heat away from high-power-density electronics in aircraft and spacecraft. For scientific cameras, copper-APG plates cool and isothermalize CCD detectors at cryogenic temperatures.
- In plane thermal conductivity at room te
- 1,700 W/m-K
- Peak thermal conductivity
- 2,800 W/m-K at approximately 150 K
- Conductivity behavior
- increases as temperature decreases
- Common encapsulant
- aluminum
- Via materials
- aluminum or copper
Lore & Background
APG is produced through a process similar to Highly Oriented Pyrolytic Graphite (HOPG), where hydrocarbon gas is heated until it breaks down into carbon. Pyrolytic graphite (PG) is grown on plates using chemical vapor deposition (CVD), then annealed at high temperature to form the more planar and uniform carbon structure of APG. Unlike HOPG, the APG annealing process does not require induced stresses, resulting in a more affordable and practical bulk material for production use.
The material's high in-plane thermal conductivity arises from covalently bonded carbon atoms arranged in a hexagonal geometry within each basal plane. Through its thickness, these planes are weakly bonded by van der Waals forces, leading to poor through-thickness thermal conductivity, stiffness, and strength. APG is also low in mass, electrically conductive, and exhibits diamagnetic properties that allow it to levitate in magnetic fields.
Because of its poor mechanical properties, APG is typically encapsulated in structural metals such as aluminum or copper. Thermal vias, often made of aluminum or copper, are sometimes inserted to transfer heat into the graphite through its thickness. Thin, flexible sheets of APG can be encapsulated in polymers or metal foils to create thermal straps.
Reader's Guide
APG is primarily used as a heat spreader for thermal management of high-end electronics, where its high in-plane conductivity efficiently spreads heat away from concentrated sources. In aerospace applications, aluminum-APG plates transfer heat away from high power density electronics in aircraft and spacecraft. For scientific cameras, copper-APG plates cool and isothermalize CCD detectors at cryogenic temperatures, leveraging APG's increasing conductivity at lower temperatures. The material's encapsulation in structural metals addresses its mechanical fragility while preserving its thermal performance. The use of thermal vias further enhances heat transfer into the graphite planes. APG's synthesis method, which avoids induced stresses, makes it more economical than HOPG for bulk production, enabling its adoption in demanding thermal management roles where weight and conductivity are critical.
Did You Know?
- APG's in-plane thermal conductivity peaks at 2,800 W/m-K at approximately 150 K.
- APG is diamagnetic and can levitate in magnetic fields.
- The material is produced by annealing pyrolytic graphite without induced stresses.
- APG is often encapsulated in aluminum or copper due to its poor mechanical properties.
Frequently Asked Questions
Who is Annealed Pyrolytic Graphite?
APG, also called Thermally Annealed Pyrolytic Graphite, is a synthetic graphite engineered specifically for outstanding lateral heat conduction. It sits in the same material family as pyrolytic carbon and standard pyrolytic graphite, sharing traits such as low mass, electrical conductivity, and diamagnetism (it can levitate in a strong magnetic field).
What are Annealed Pyrolytic Graphite's powers and role in a cooling stack?
Its layered, anisotropic structure delivers roughly 1,700 W/m·K of in-plane thermal conductivity at room temperature, climbing to about 2,800 W/m·K near 150 K. In practice it acts as a thin, ultra-efficient heat-spreading interposer, typically encapsulated in aluminum with aluminum or copper vias to bridge the vertical gap.
What is Annealed Pyrolytic Graphite's greatest weakness?
Because conductivity is strongly anisotropic, heat flows extremely well laterally but very poorly through the sheet's thickness. This means APG cannot by itself channel heat vertically, so designers must pair it with metal vias or a copper/aluminum cap to complete the thermal path.
Why is Annealed Pyrolytic Graphite important to the cooling and modding community?
Its laminate structure stays thermally stable across a wide temperature window, and its conductivity actually rises as temperature drops, giving it reliable performance in both hot and cold operating regimes. Fans and modders favor it as a lightweight, passive layer that outperforms plain copper for spreading heat across a broad surface area.
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