THERMAL CONDUCTIVITY TESTING

Apply For Services

WHAT IS THERMAL CONDUCTIVITY TESTING?

Thermal Conductivity Testing measures a material's ability to conduct heat, which is crucial for understanding its thermal performance in various applications, such as electronics, construction, automotive, and aerospace. Thermal conductivity (k) indicates how easily heat flows through a material, and it's expressed in units of W/(m·K), or watts per meter-kelvin. Materials with high thermal conductivity, like metals, transfer heat quickly, whereas those with low thermal conductivity, like insulators, resist heat flow.

OBJECTIVES OF THERMAL CONDUCTIVITY TESTING

1.    Material Selection: Helps in selecting appropriate materials for heat dissipation or insulation in various applications.

2.    Quality Control: Ensures materials consistently meet required thermal performance standards.

3.    Safety Assurance: Verifies that materials will perform safely at high or low temperatures.

4.    Regulatory Compliance: Confirms that materials meet industry and legal standards for thermal properties.

5.    Optimization: Helps engineers and designers create products that can effectively manage heat.

TYPES OF THERMAL CONDUCTIVITY TESTING

Thermal conductivity testing can be done through steady-state or transient methods, each suitable for different materials and applications.

1. Steady-State Methods

Steady-state methods measure thermal conductivity by applying a constant heat source and observing the temperature change over time once equilibrium is reached.

• Guarded Hot Plate Method
• Purpose: Measures the thermal conductivity of solid materials, especially insulators.
• Method: A sample is placed between a heated and a cooled plate, and the temperature gradient across the sample is measured once steady-state heat flow is achieved.
• Applications: Common for testing insulation materials like foam, fiberglass, and mineral wool.
• Heat Flow Meter Method
• Purpose: Measures the thermal conductivity of a sample by assessing heat flux.
• Method: Heat flows through the sample between two plates, and the heat flux and temperature difference are used to calculate thermal conductivity.
• Applications: Commonly used for materials like glass, polymers, and building insulation.
• Comparative Cut Bar Method

Purpose: Suitable for testing materials with moderate to high thermal conductivity.

Method: The sample is placed between two reference materials of known conductivity, and the temperature drop across all three sections is used to determine the sample's thermal conductivity.

Applications: Frequently used for metals, ceramics, and composites.

2. Transient Methods

Transient methods measure the material’s thermal response over a short period, providing faster results than steady-state methods and suitable for a wider range of materials.

• Hot Disk (Transient Plane Source) Method

• Purpose: Measures thermal conductivity in a range of materials, from solids to liquids.
• Method: A thin, disk-shaped sensor is placed between two sample halves, and a pulse of heat is applied. The temperature increase in the sensor is monitored to determine thermal conductivity.
• Applications: Used for polymers, metals, liquids, and foams, especially where high precision is required.
• Laser Flash Method
• Purpose: Measures the thermal diffusivity of materials with high thermal conductivity, such as metals and ceramics.
• Method: A pulse of laser energy is applied to the sample's front surface, and the temperature rise on the rear surface is recorded. Thermal conductivity is calculated from the thermal diffusivity, density, and specific heat capacity.
• Applications: Common for high-temperature materials, especially in aerospace and automotive industries.
• Transient Hot-Wire Method
• Purpose: Used to measure the thermal conductivity of fluids and gases.
• Method: A heated wire is placed in the sample, and the heat dissipation rate is measured to determine thermal conductivity.
• Applications: Suitable for fluids like oils, gases, and molten polymers.
• Thermocouple-Based Needle Probe Method
• Purpose: Measures the thermal conductivity of soils, rocks, and granular materials.
• Method: A needle probe containing a heater and thermocouple is inserted into the sample, and thermal conductivity is calculated from temperature changes.
• Applications: Common in geotechnical engineering, environmental science, and construction.

EQUIPMENT USED IN THERMAL CONDUCTIVITY TESTING

1.    Guarded Hot Plate Apparatus: Used for steady-state thermal conductivity testing, especially for insulating materials.

2.    Heat Flow Meters: Devices that measure the heat flux through a sample between two plates.

3.    Laser Flash Apparatus: Equipment for measuring thermal diffusivity in high-conductivity materials.

4.    Hot Disk Sensors: Thin, disk-shaped sensors used in transient plane source methods.

5.    Thermal Conductivity Meters: Multi-functional devices designed for quick, transient thermal conductivity measurements in both solid and liquid materials.

6.    Needle Probes: Thermocouple-equipped probes used for measuring the thermal conductivity of soils and granular materials.

STANDARDS AND GUIDELINES FOR THERMAL CONDUCTIVITY TESTING

To ensure consistency, reliability, and accuracy, several industry standards guide thermal conductivity testing:

• 1.    ASTM Standards:

o    ASTM C518: Standard for measuring thermal conductivity by the heat flow meter method.

o    ASTM D5470: Standard for measuring thermal impedance of thermal interface materials.

o    ASTM E1952: Standard for thermal conductivity and diffusivity by the laser flash method.

o    ASTM C177: Standard for guarded-hot-plate thermal conductivity testing.

• 2.    ISO Standards:

o    ISO 22007: Series of standards on thermal conductivity testing for polymers using different methods.

o    ISO 8302: Guarded hot plate method for steady-state thermal conductivity measurement.

o    ISO 8894-2: Hot wire method for refractory materials’ thermal conductivity.

• 3.    EN Standards:

o    EN 12667: Thermal performance of building materials using the guarded hot plate method.

o    EN 12939: Large-sized samples of building materials’ thermal conductivity measurement.

• 4.    Other Industry Standards:

o    IEEE 442: For thermal conductivity of insulating materials in electrical equipment.

o    BS 874-2: British standard for thermal conductivity measurement using steady-state methods.

APPLICATIONS OF THERMAL CONDUCTIVITY TESTING

Thermal conductivity testing is essential across various industries, such as:

1.    Electronics: Used to evaluate materials for thermal management in components, circuit boards, and heat sinks to prevent overheating.

2.    Construction: Tests insulation materials like foam, glass wool, and    aerogels to ensure they meet energy efficiency standards and provide adequate thermal resistance.

3.    Aerospace and Automotive: Ensures materials like metals, composites, and ceramics can handle high thermal loads, helping with heat dissipation and thermal management.

4.    Manufacturing: Used for thermal interface materials (TIMs) in machinery and tools to enhance performance and durability.

5.    Oil and Gas: Tests thermal properties of drilling fluids and pipeline materials exposed to extreme temperatures.

6.    Power Generation: Assesses the thermal properties of materials in turbines, engines, and heat exchangers to optimize energy efficiency.

7.    Food and Pharmaceuticals: Used to study the thermal properties of packaging materials and storage containers to ensure temperature stability.

IMPORTANCE OF THERMAL CONDUCTIVITY TESTING

Energy Efficiency: Helps optimize materials for insulation, reducing energy costs and environmental impact.

Safety Assurance: Ensures materials meet thermal stability requirements, preventing failures in high-heat environments.

Improved Product Performance: Enables better thermal management in products like electronics and automotive components.

Cost Savings: Helps manufacturers avoid costly failures by ensuring materials perform as expected in thermal applications.

Regulatory Compliance: Ensures that products meet local and international standards for thermal insulation, safety, and efficiency.

CHALLENGES IN THERMAL CONDUCTIVITY TESTING

Complexity in Testing Non-Homogeneous Materials: Materials like composites can have varying conductivity across different sections, making uniform measurements challenging.

Temperature Dependency: Thermal conductivity often changes with temperature, requiring testing across a range of temperatures for accurate results.

Sample Preparation: Preparing samples to meet the precise requirements of each testing method can be difficult, especially for brittle or high-temperature materials.

Time-Consuming Steady-State Methods: Steady-state methods can be slow, especially for low-conductivity materials, as they require thermal equilibrium.

Environmental Control: Testing often requires controlled environments, especially for transient methods, to reduce heat loss and ensure accuracy.

SUMMARY

Thermal conductivity testing is essential for assessing a material's heat-transfer properties, supporting applications in electronics, construction, aerospace, automotive, and many other fields. Various methods, from steady-state to transient techniques, enable the testing of different materials, from solids to fluids and gases. Each method has specific applications, advantages, and limitations, and standards guide testing procedures to ensure reliable results. Thermal conductivity testing plays a vital role in product design, safety, energy efficiency, and regulatory compliance, helping engineers and designers make informed decisions for thermal management in diverse applications.