In advanced thermal field applications, carbon-based materials are exposed to extreme temperatures, vacuum or inert atmospheres, and repeated thermal cycles.

Their performance is therefore determined not only by cleanliness and microstructure, but also by how they transfer heat, conduct electricity, and respond to thermal expansion.

Three key thermal-electrical parameters are particularly important:

Thermal Conductivity

Electrical Resistivity

Coefficient of Thermal Expansion (CTE)

These properties directly influence:

Temperature uniformity

Heating efficiency

Heater impedance matching

Thermal stress

Dimensional stability

However, the optimal value is not necessarily the highest or lowest value.

The correct target depends on the material, component, heat-flow direction, furnace design, and operating conditions.

1. Thermal Conductivity: Controlling Heat Transfer

Definition

Thermal conductivity describes the ability of a material to transfer heat.

For carbon and graphite materials, thermal conductivity can be calculated from thermal diffusivity, density, and specific heat:

λ = α × ρ × Cp

Where:

λ = Thermal conductivity

α = Thermal diffusivity

ρ = Density

Cp = Specific heat capacity

ASTM C714 covers the thermal pulse method for measuring thermal diffusivity of carbon and graphite materials. It defines thermal diffusivity in relation to thermal conductivity, density, and specific heat.

Therefore, thermal conductivity data should always be considered together with the test method and test temperature.

2. Higher Thermal Conductivity Is Not Always Better

Different thermal field components have different thermal requirements.

For example:

For heaters, susceptors and other heat-transfer components, stable and relatively high thermal conductivity can help reduce local temperature differences and hot spots.

For insulation materials such as soft felt and rigid felt, the objective is generally the opposite.

Lower thermal conductivity helps reduce heat loss and furnace energy consumption.

This leads to an important principle:

The "best" thermal conductivity depends on the function of the component.

3. Thermal Conductivity of Graphite and C/C

Isostatic graphite commonly exhibits thermal conductivity values ranging from several tens to more than 100 W/m·K, depending on grade and application.

Typical technical data from different graphite manufacturers show values in the approximate range of:

70–140 W/m·K

for various grades.

C/C composites require additional consideration because their thermal conductivity can be strongly anisotropic.

Typical technical data indicate approximately:

X-Y direction: 70–80 W/m·K

Z direction: 1–10 W/m·K

The significant difference between directions demonstrates the importance of fiber architecture.

For C/C components:

The fiber orientation should be designed according to the expected heat-flow direction.

Ignoring this anisotropy can lead to unexpected temperature gradients and dimensional deformation.

4. Thermal Conductivity Uniformity Matters

In actual furnace applications, the average thermal conductivity of a material is only part of the story.

Local or batch-to-batch variation can also affect thermal field stability.

Non-uniform thermal conductivity may contribute to:

Temperature field drift

Abnormal temperature gradients

Local hot spots

Local erosion or oxidation

Heat shield deformation

Increased energy consumption

For this reason, advanced thermal field specifications should consider more than a single nominal value.

Important parameters may include:

Test temperature

Measurement direction

Sampling position

Within-batch variation

Batch-to-batch variation

Thermal conductivity retention after furnace cycling

5. Electrical Resistivity: Controlling Heating Behavior

Definition

Electrical resistivity describes the resistance of a material to the flow of electric current.

For carbon and graphite products, ASTM C611 covers the measurement of electrical resistivity of manufactured carbon and graphite products at room temperature.

For resistance-heating components, electrical resistivity directly affects electrical and thermal behavior.

The relationship between power, current and resistance can be expressed as:

P = I²R

Where:

P = Electrical power

I = Current

R = Resistance

In practical furnace systems, however, material resistivity must be considered together with component geometry, operating temperature and power-supply characteristics.

6. Why Resistivity Uniformity Is Critical for Heaters

For a graphite heater, the average electrical resistivity does not tell the complete story.

If resistivity varies significantly within the component, current distribution can become non-uniform.

This can create localized heating and thermal stress.

Potential consequences include:

Hot spots

Local overheating

Accelerated oxidation

Cracking

Local material loss

Heater failure

Therefore:

For heating components, resistivity uniformity can be more important than the average resistivity itself.

Typical room-temperature electrical resistivity for many isostatic graphite grades is approximately:

11–17 μΩ·m

Specific values depend on graphite grade, structure and application.

7. Monitoring Resistance During Furnace Service

Initial electrical resistance is only a starting point.

For critical heater applications, resistance can be tracked throughout the component's service life:

Initial Resistance

↓

Resistance After Baking

↓

Resistance After N Furnace Cycles

↓

Resistance at End of Service Life

An increase in resistance may indicate:

Oxidation

Cracking

Reduction in effective cross-sectional area

Material degradation

Poor electrical contact

Connection deterioration

Therefore, resistance drift can serve as a practical indicator of component condition and remaining service life.

8. Coefficient of Thermal Expansion (CTE)

Definition

The coefficient of thermal expansion describes the dimensional change of a material as temperature changes.

It can be expressed as:

α = ΔL / (L₀ × ΔT)

Where:

α = Coefficient of thermal expansion

ΔL = Change in length

L₀ = Initial length

ΔT = Temperature change

In high-temperature thermal fields, CTE is directly related to:

Thermal stress

Assembly clearance

Warpage

Cracking

Dimensional stability

9. CTE Mismatch: A Common Source of Thermal Stress

Thermal field systems often contain multiple materials.

For example:

Graphite

C/C composites

SiC coatings

Quartz

SiC crystals

Metal fastening structures

These materials may have different coefficients of thermal expansion.

During repeated heating and cooling, differences in expansion and contraction can generate internal stress.

Over time, this may lead to:

Warpage

Cracking

Delamination

Dimensional drift

Clearance changes

Premature component failure

Therefore:

Material compatibility should be evaluated not only by chemical compatibility, but also by CTE compatibility.

10. Typical CTE Values

For isostatic graphite, typical CTE values are approximately:

4.5–6.2 × 10⁻⁶/K

depending on material grade and temperature range.

C/C composites can exhibit significantly different behavior because of their fiber architecture.

Through fiber orientation and structural design, C/C can achieve very low or even negative CTE values in the X-Y direction.

However, the Z direction can have a considerably higher CTE, potentially around:

6–8 × 10⁻⁶/°C

This highlights an important consideration:

For anisotropic materials, CTE must always be reported together with the measurement direction.

11. CTE Should Be Reported Over a Temperature Range

A single CTE value measured near room temperature may not adequately represent the behavior of a component operating at extremely high temperatures.

For advanced thermal field applications, CTE should preferably be reported over defined temperature ranges, such as:

RT–500°C

RT–800°C

RT–1600°C

This is particularly important for large components such as:

Heat shields

Guide cylinders

Insulation cylinders

Susceptors

C/C structural components

For large-scale thermal field components, both CTE anisotropy and batch-to-batch variation should be considered.

12. Three Properties, One Thermal Field

Thermal conductivity, electrical resistivity and CTE may be measured separately, but their effects are closely connected during furnace operation.

Thermal Conductivity

Heat Transfer

↓

Temperature Distribution

↓

Thermal Field Uniformity

Electrical Resistivity

Current Distribution

↓

Power Generation

↓

Heating Efficiency

CTE

Thermal Expansion

↓

Thermal Stress

↓

Dimensional Stability

Together, these properties determine how a carbon-based material behaves under actual operating conditions.

13. From Material Data to Real Furnace Performance

A material datasheet may provide:

Thermal conductivity

Electrical resistivity

CTE

But these numbers alone do not guarantee stable furnace performance.

The same material can behave differently depending on:

Component geometry

Material orientation

Operating temperature

Heat-flow direction

Assembly configuration

Thermal cycling history

Therefore, advanced evaluation should move beyond:

"What is the material's nominal value?"

and ask:

"How stable is this property under actual operating conditions?"

This is particularly important for high-temperature semiconductor, photovoltaic and SiC applications where even small changes in temperature distribution can affect process stability and product quality.

14. What Should Be Included in a Technical Specification?

For advanced carbon-based thermal field materials, specifications should clearly define the testing conditions.

Thermal Conductivity

Specify:

Test temperature

Test method

Measurement direction

Sampling position

Within-batch variation

Thermal conductivity retention after furnace cycling

Electrical Resistivity

Specify:

Test temperature

Measurement direction

Initial resistance

Resistance after baking

Resistance after defined furnace cycles

Resistance drift

CTE

Specify:

Temperature range

Measurement direction

CTE value

CTE anisotropy

Batch-to-batch variation

This creates a more complete picture of material performance than a single room-temperature datasheet value.

Conclusion

Thermal-electrical properties are fundamental to the performance of carbon-based thermal field materials.

Thermal conductivity determines how heat moves through the thermal field.

Electrical resistivity determines how electrical energy is converted into heat.

CTE determines how materials respond dimensionally to temperature changes.

But the most important point is that these properties should not be evaluated in isolation.

For advanced thermal field applications, the evaluation should consider:

Value + Uniformity + Direction + Temperature + Service History

Only by connecting material properties with actual furnace conditions can we better understand the long-term performance of thermal field components.

In the next article:

Core Performance Metrics of Carbon-Based Thermal Field Materials (Part IV): Reliability - From Thermal Shock to Long-Term Furnace Life

XINGHUI MATERIALS

Advanced Carbon-Based Thermal Field Solutions for Semiconductor, Photovoltaic and High-Temperature Applications.