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Thermal conductivity testing methods

Thermal conductivity testing methods

There are many different ways to test the thermal conductivity of materials. Some methods are more accurate than others, and some are more suitable for certain materials than others. In this blog post, we will take a look at some of the most common thermal conductivity testing methods and discuss their advantages and disadvantages.

Introduction


Thermal conductivity is a material property that is widely used in the coming field of nanotechnology, particularly in the area of thermal management. In simple terms, it is a measure of a material’s ability to conduct heat. Thermal conductivity testing is the process of measuring this property.

There are three main methods used to measure thermal conductivity: the guarded hot plate method, the transient plane source (TPS) method, and the flash method. Each of these methods has its own advantages and disadvantages, which will be discussed in more detail below.

The guarded hot plate method is perhaps the most common method used to measure thermal conductivity. In this method, a thin layer of the material being tested is placed between two metal plates. One of the plates is heated, while the other remains at a constant temperature. The rate at which heat flows from the hot plate to the cold plate is then measured, and from this, the thermal conductivity of the material can be calculated.

One advantage of this method is that it can be used to test materials with very low thermal conductivity, as long as they are sufficiently thin. However, it can be difficult to achieve accurate results with very thick samples, due to heat losses through the metal plates. In addition, this method can be time-consuming and expensive, due to the need for specialized equipment.

The transient plane source (TPS) method consists of passing an electrical current through a cylindrical probe that is inserted into drill core or other samples of rock. The resulting heat flow is then measured with thermocouples placed around the circumference of the probe. This information is then used to calculate the thermal conductivity of the material being tested.

One advantage of this method is that it can be used to test samples that are too large or too thick for other methods such as guarded hot plate or flash methods. However, like those methods, it can be expensive and time-consuming due to the need for specialized equipment. In addition, this technique cannot be used on materials with low electrical resistivity (such as graphite).

The flash method uses rapid heating by an electric arc or laser to produce a thin layer of melt on the surface of the material being tested. The thermal conductivity is then measured by monitoring the change in temperature of a cooling copper disk as heat flows into it from the melt. This method is fast and relatively inexpensive, but it has limited applicability because it can only be used for materials that have melting points lower than that of copper( 1084 °C ) . For this reason, it is mostly used for testing polymers and other low-melting point materials.

The Basics of Thermal Conductivity Testing


Thermal conductivity is a material property that describes how well a material conducts heat. Thermal conductivity testing is performed to measure a material’s ability to conduct heat. The test methods used to measure thermal conductivity can vary depending on the type of material being tested and the desired level of accuracy.

Thermal conductivity testing is commonly performed on insulation materials, such as fibreglass, cellulose, and foam. The thermal conductivity of these materials is typically measured in watts per meter per Kelvin (W/m-K). Thermal conductivity values can range from 0.03 W/m-K for fiberglass to 0.17 W/m-K for cellulose.

There are three main methods used to measure the thermal conductivity of a material: hot wire, guarded hot plate, and laser flash. Each of these methods has its own advantages and disadvantages.

Hotwire thermal conductivity testing is typically used for thin materials with high thermal conductivities, such as metals. The hot wire method uses a simple apparatus that consists of a heated wire suspended in the air above the material being tested. The temperature of the wire is measured using a calibrated thermocouple. The heat flowing through the wire is then calculated using the following equation:

Q = kA(ΔT/ΔL)

where Q is the heat flow rate (W), k is the thermal conductivity (W/m-K), A is the cross-sectional area of the wire (m2), ΔT is the temperature difference between the two ends of the wire (°C), and ΔL is the length of the wire (m).

Guarded hot plate thermal conductivity testing is typically used for thicker materials with lower thermal conductivities, such as insulation materials. The guarded hot plate method uses an apparatus that consists of two plates: a heated plate and an unheated “guard” plate. The material being tested is placed between the two plates, and the temperature difference between them is measured using a calibrated thermocouple. The heat flow through the material can then be calculated using the following equation:

Q = kA(ΔT/Δx)t

where Q is the heat flow rate (W), k is the thermal conductivity (W/m-K), A is the surface area of one plate (m2), ΔT is the temperature difference between th

Different Thermal Conductivity Testing Methods


Thermal conductivity is a very important property when choosing the right material for an application. It is a measure of a material’s ability to conduct heat and is commonly used in the construction, automotive and electronics industries, among others.

There are a number of different methods used to measure thermal conductivity, each with its own advantages and disadvantages. The most common methods are the guarded hot plate method, the dual Slab method and the flash method.

The guarded hot plate method is the most widely used method for measuring thermal conductivity. It is generally considered to be the most accurate method, but it can be expensive and time-consuming.

The dual Slab method is a newer method that is growing in popularity due to its accuracy and simplicity. This method uses two thin sheets of material separated by a thin gap. Heat is applied to one sheet and the temperature difference between the two sheets is measured.

The flash method is the simplest and most commonly used thermal conductivity test method. A sample of material is heated for a short period of time and then cooled rapidly. The temperature change of the sample is then measured and used to calculate the thermal conductivity.

Why is Thermal Conductivity Testing Important?


Thermal conductivity testing is essential for many industries, including the automotive, aerospace, and construction sectors. By understanding the thermal conductivity of materials, engineers can design products that dissipate heat efficiently and safely. For example, in the automotive industry, thermal conductivity testing is used to assess the ability of materials to dissipate heat generated by brake pads and other components. In the aerospace industry, thermal conductivity testing is used to evaluate the potential for materials to overheat during flight.

Thermal conductivity testing can be performed using several different methods, each with its own advantages and disadvantages. The most common methods are steady-state heat flow measurement and transient hot wire measurement.

Applications of Thermal Conductivity Testing

Thermal conductivity is one of the most important properties of materials used in the design and construction of homes, buildings, electronic equipment, and a variety of other products. The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly used in the construction industry to determine the insulation value of materials. It is also used in the electronics industry to determine how well a material can dissipate heat.

How to Perform Thermal Conductivity Testing


Thermal conductivity testing is a fundamental thermal analysis technique that quantifies a material’s ability to transfer heat. The test involves applying a known temperature gradient across a material sample and measuring the heat flow through the material. Because thermal conductivity is a product of both the intrinsic properties of the material and the prevailing test conditions, it is important to carefully control these parameters during testing.

There are several methods available for performing thermal conductivity testing, each with its own advantages and disadvantages. The most common methods are the guarded hot plate method, the transient plane source (TPS) method, and the flash method. In this article, we will briefly discuss each of these methods and their key applications.

Guarded Hot Plate Method
The guarded hot plate method is perhaps the most commonly used thermal conductivity testing method. It is well suited for measuring a wide range of materials, from low-density foams to high-density metals. The key advantage of this method is its versatility – it can be used to test samples of almost any size, shape, or geometry. Additionally, the guarded hot plate method can be adapted for use in a wide variety of environments, from vacuum chambers to highly humid atmospheres.

The setup for a guarded hot plate test typically consists of three parts: a heater element, a sensor element, and a guard ring (Figure 1). The heater element and sensor element are usually made from platinum or another high-thermal-conductivity metal, such as silver. The guard ring surrounds the heater and sensor elements and helps to minimize heat loss from convection and radiation.

The sample to be tested is placed on top of the heater element, and a known temperature gradient is applied across the sample by heating the element to a high temperature and cooling the sensor element to a lower temperature. The resulting heat flow through the sample is then measured by monitoring the temperature change of the sensor element over time. By knowing the applied temperature gradient and measuring the resulting heat flow, it is possible to calculate the thermal conductivity of the sample using standard equations[1].

Transient Plane Source (TPS) Method
The transient plane source (TPS) method[2] is similar to the guarded hot plate method in that it also uses platinum or other high-conductivity elements for both heating and sensing purposes. However, in contrast to guarded hot plates which have surrounding guard rings that minimize convective heat losses, TPS sensors have an embedded heater element that allows them to minimize both convective and radiative heat losses. Additionally, TPS sensors have very thin geometries (on the order of millimeters), which allow them to quickly come into thermal equilibrium with their surroundings after being inserted into a material sample[3]. As such, TPS sensors are well suited for measuring materials with low thermal conductivities (i.e., insulation materials) or for testing samples with complex geometries where it would be difficult to place a larger sensor in contact with all exposed surfaces[4].

Figure 2: A typical transient plane source sensor[5]

To perform a TPS test[6], an operator first inserts a TPS sensor into drilled holes or cores taken from larger bulk samples. Once inserted, an electrical current is passed through embedded heating wires in order to bring the sensor up to its desired operating temperature[7]. After allowing sufficient time for the sensor and bulk sample to come into equilibrium,[8]thermal couples measure the temperatures of both the sensor tip and an areaof exposed surface surrounding the tip.[9] These measurements allow for the determination of the okes’ statistical parameter(also known as thermal diffusivity),from which thermal conductivity can be calculated using standard equations.[10],[11]

Flash Method

The flash method is an important variant of the guarded hot plate and TPS methods that is better suited for rapidly measuring thermal conductivity at elevated temperatures (500°C). The basic principle of the flash method is similar to both other methods in that it uses a heat flux sensor and temperature differences to infer thermal conductivity; however, unlike the other two methods, the flash method involves briefly (on the order of milliseconds) applying a high heat flux to surfaces to be measured in order to determine a resistance to the flow of fluid through porous media or other highly resistive samples. This resistance can be then used to calculate thermal conductivities, compressibilities, and other material properties.

Tips for Accurate Thermal Conductivity Testing

There are three main things to consider when trying to get accurate thermal conductivity readings: sample size, contact area, and uniformity of temperature.

Sample size: it is important to have enough material so that the heat can flow evenly through the sample and not be affected by sporadic warmer or cooler areas. Too small of a sample will give you anomalous readings. Generally, a minimum of 3 in x 3 in (7.6 cm x 7.6 cm) is ideal, but larger is better.

Contact area: in order to get an accurate reading, it is important that the entire surface area of the sample is in contact with the sensor. If there are gaps or air pockets, the heat will flow around those areas and not provide an accurate reading.

Uniformity of temperature: for an accurate reading, it is important that the temperature of the sample is uniform throughout. If there are areas that are significantly warmer or cooler, it will skew the reading. The best way to achieve uniformity is to use a controlled heating/cooling system such as a furnace or oven.

Conclusion

Thermal conductivity is a measure of a material’s ability to conduct heat. It is most commonly measured in watts per meter per Kelvin (W/(m·K)), but can also be expressed in British thermal units per hour per square foot per degree Fahrenheit (Btu/(h·ft2·°F)) or joules per second per meter per kelvin (J/(s·m·K)). Thermal conductivity is an important property for materials that are used in heat transfer applications, such as insulation.

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