Determination of Thermal Conductivity of Artificial Graphite Heat Dissipation Sheets

Artificial graphite heat dissipation sheets have become the dominant thermal management material in smartphones, tablets, and high-power electronics, yet their performance can only be verified through accurate thermal conductivity measurement. This article explains how the in-plane and through-plane thermal conductivity of synthetic graphite films is determined using internationally recognized methods such as the laser flash technique (ASTM E1461, ISO 22007-4). It covers sample preparation, testing items, applicable standards, calculation principles, and typical application scenarios for laboratory testing services.

Technical Overview

Artificial graphite heat dissipation sheets, also known as synthetic graphite films or graphite heat spreaders, are manufactured by carbonizing and graphitizing polyimide (PI) precursor films at temperatures of approximately 2,000 to 3,000 degrees Celsius. The resulting material possesses a highly oriented layered crystal structure, which gives it exceptional thermal spreading capability combined with low weight, thin profile, and mechanical flexibility.

Because of this layered structure, the thermal conductivity of artificial graphite is strongly anisotropic. The in-plane (X-Y direction) thermal conductivity typically ranges from 700 to 1,950 W/(m·K), while the through-plane (Z direction) value is usually only 5 to 20 W/(m·K). Accurate determination of both directional values is essential for thermal simulation, product design, incoming material inspection, supplier qualification, and quality control during mass production.

• Highly anisotropic heat transfer: in-plane versus through-plane conductivity must be measured separately
• Thin film format: typical thicknesses of 10 to 100 micrometers require special sample preparation
• Temperature-dependent behavior: conductivity can be characterized from sub-ambient to elevated temperatures
• Standardized testing: results must be traceable to internationally recognized test methods

Testing Samples

Single-Layer Synthetic Graphite Films

Pure artificial graphite films without adhesive or protective layers, with thicknesses typically between 10 and 100 micrometers. These samples are tested either as stacked laminates (through-plane) or as cross-plied strip assemblies (in-plane) to obtain measurable specimen thickness.

Multilayer and Laminated Graphite Sheets

Commercial heat dissipation sheets that combine graphite with double-sided tape, PET insulation films, or release liners. The complete laminate can be tested to evaluate the effective thermal performance of the product as actually installed in a device.

Graphite Composite Structures

Graphite laminated with copper foil, aluminum foil, or polymer substrates, as well as graphite-coated components used in enhanced thermal modules. Composite samples are tested to verify the contribution of the graphite layer to overall heat spreading.

Natural Graphite Sheets for Comparison

Natural graphite-based sheets can be tested under identical conditions to provide a benchmark comparison with artificial graphite products, which typically show superior in-plane conductivity.

Testing Items

Comprehensive thermal characterization of artificial graphite sheets generally includes the following measurable parameters:

• In-plane thermal diffusivity (X-Y direction), units mm²/s
• Through-plane thermal diffusivity (Z direction), units mm²/s
• Specific heat capacity, determined by differential scanning calorimetry (DSC)
• Density, determined by geometric or Archimedes methods
• In-plane thermal conductivity, calculated and reported in W/(m·K)
• Through-plane thermal conductivity, calculated and reported in W/(m·K)
• Thermal resistance of laminated assemblies, reported in K·cm²/W
• Temperature-dependent thermal conductivity profiles across a defined temperature range

Testing Methods

Laser Flash Method (ASTM E1461 / ISO 22007-4)

The laser flash method is the industry-standard technique for graphite films. A short energy pulse heats the front face of the specimen, and an infrared detector records the temperature rise on the rear face. From the resulting curve, the thermal diffusivity is calculated. Thermal conductivity is then derived from the fundamental relationship: thermal conductivity equals thermal diffusivity multiplied by density and specific heat capacity.

• Through-plane testing: multiple film layers are stacked and lightly bonded into a disc of adequate thickness, typically 0.5 to 3 mm, to ensure reliable rear-face signal detection
• In-plane testing: strips of film are stacked and rotated 90 degrees alternately (cross-plied), or a dedicated in-plane sample fixture is used, so that heat flow along the sheet plane is captured
• A thin graphite spray coating may be applied to enhance energy absorption and infrared emissivity of the specimen surfaces
• Careful stacking under controlled pressure minimizes interfacial air gaps that would otherwise distort the measured diffusivity

Transient Plane Source Method (ISO 22007-2)

The transient plane source (hot disk) technique uses a spiral sensor sandwiched between two pieces of the sample. A controlled electrical heating pulse is applied, and the sensor simultaneously records the temperature response, allowing direct determination of thermal conductivity. With appropriate anisotropic analysis models, both in-plane and through-plane conductivity can be extracted from a single measurement, making the method attractive for rapid screening.

Guarded Heat Flow Meter Method (ASTM E1530 / ASTM C518)

Steady-state heat flow methods establish a one-dimensional temperature gradient across a specimen clamped between temperature-controlled plates. These methods are well suited to thicker laminated graphite assemblies and thermal interface products, and they provide direct measurement of thermal resistance and effective through-plane conductivity.

Specific Heat Capacity by DSC (ASTM E1269 / ISO 11357-4)

Specific heat capacity is measured by differential scanning calorimetry using the sapphire reference method. This value is a required input for converting laser flash diffusivity data into thermal conductivity, so its accuracy directly influences the final reported result.

Applications

Thermal conductivity test data for artificial graphite sheets directly supports engineering decisions across many industries:

• Smartphones, tablets, and wearables: spreading heat from processors, cameras, and fast-charging circuits
• Laptops, GPUs, and gaming devices: managing heat flux from high-performance silicon
• OLED and LCD display modules: equalizing panel temperature to prevent brightness mura and burn-in
• 5G base stations and telecommunication hardware: dissipating heat from RF power amplifiers
• Electric vehicle battery packs and power electronics: thermal spreading for IGBT and SiC modules
• LED lighting and automotive luminaires: extending component lifetime through uniform cooling
• Aerospace and defense electronics: lightweight thermal control in constrained environments

Reliable laboratory data enables design engineers to select the correct graphite grade, allows suppliers to certify material consistency between production lots, and supports failure analysis when devices overheat in the field.

FAQ

Why are the in-plane and through-plane thermal conductivity values so different?

Graphite has a layered crystal structure. Heat is transported very efficiently along the basal planes but poorly perpendicular to them, so the in-plane value may exceed the through-plane value by a factor of one hundred. Both directions must be tested and reported for a complete material description.

What sample size and quantity are required?

For laser flash testing, discs of approximately 10 to 25 mm diameter are typical. Because single films are very thin, customers usually supply sheets large enough to cut and stack the required number of layers, often a minimum area of 100 mm by 100 mm per sample.

What is the measurement uncertainty?

With properly prepared specimens, thermal diffusivity by laser flash is typically reported with an uncertainty of about 3 to 5 percent, while calculated thermal conductivity, which combines diffusivity, density, and specific heat data, is generally within 5 to 8 percent.

Can testing be performed at elevated or low temperatures?

Yes. Laser flash instruments can characterize graphite films from approximately -50 degrees Celsius up to 500 degrees Celsius, generating a full temperature-dependent thermal conductivity curve when required.

Which method should be chosen for a given project?

The laser flash method is preferred for thin graphite films and for temperature-dependent studies, while the guarded heat flow meter is better suited to thicker laminates and finished thermal interface assemblies. Testing laboratories can recommend the optimal method based on sample format, thickness, and end-use requirements.