Hello, I’m Thermo Man.

Today, I would like to discuss a commonly asked question about methods for measuring thermal conductivity: the difference between the "steady-state method" and the "transient method."

A frequently asked question is, “If the same material is measured using both methods, will the thermal conductivity values be the same?”

In conclusion, the results can be the same in some cases, and different in others. The reasons are explained in detail at the end of this article, but if you want to know right away, please refer to the section titled "Do the Results of Steady-State and Transient Methods Differ?"

First, let's explain the fundamental differences between these two methods.

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The Fundamental Differences Between the Steady-State Method and the Transient Method

There are two main methods for measuring thermal conductivity: the "steady-state method" and the "transient method."

The steady-state method involves applying a constant temperature gradient to a sample and measuring the thermal conductivity once the system has stabilized.

Simply put, one side of the sample is heated, and the other side is cooled. After waiting for the temperature to stabilize, the temperatures at different points, the heat flux, and the thickness of the sample are measured to calculate the thermal conductivity.

**Advantages**: The thermal conductivity can be directly measured.  

**Disadvantages**: Larger samples are generally required, and the measurement process takes time.

This method is commonly used for measuring insulation materials and building materials. It is also often used for materials with high thermal conductivity, such as TIM (Thermal Interface Materials).

The transient method measures thermal conductivity by applying a temporary heat flow to the sample and analyzing its temperature response.

In simple terms, energy that changes over time (such as a pulse) is applied to the surface of the sample, and the temperature change on the opposite side is measured. There are several variations of this method, including pulse heating, periodic heating, and step heating, with the most widely used being the "flash method."

**Advantages**: Small samples can be measured quickly.  

**Disadvantages**: Thermal conductivity cannot be directly measured, and additional information such as specific heat capacity and density is required.

The transient method is commonly used for metals and ceramics. It also covers materials with extremely high thermal conductivity, such as graphite sheets and diamonds.

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When the material is homogeneous and the effects of thermal contact resistance are negligible, the measurement results typically do not vary significantly. However, the following points should be noted:

- **For low thermal conductivity materials**: The steady-state method tends to be more reliable. This is because the high thermal resistance of the material makes it easier to measure temperature differences accurately. Additionally, the influence of thermal contact resistance is minimized. In the transient method, rapid heating (such as using a laser pulse) may cause damage to the sample, and temperature gradients within the sample can lead to uneven thermal conductivity.

- **For high thermal conductivity materials**: The transient method is generally more suitable. For materials with high thermal conductivity, heat transfer occurs very quickly, making the transient method more advantageous as it only requires measurements of time and distance. In the steady-state method, measuring temperature differences becomes difficult, and the influence of thermal contact resistance is greater.

The size and uniformity of the sample are also important factors. In general, the steady-state method requires larger samples. If the sample is homogeneous, there is no issue, but if the properties vary by location, the thermal conductivity may differ depending on the portion of the sample that is tested.

For composite materials, it is more likely that the results will differ between the two methods.

- The transient method is fundamentally designed for homogeneous materials, with the analysis method assuming the measurement of thermal diffusivity in such materials. Although there are analysis methods for composite materials, their application ranges are often limited, and sufficient corrections may not always be possible.

- In contrast, the steady-state method measures actual temperature differences and heat flux, meaning it is more likely to provide accurate results even for composite materials. If the thermal resistance and thickness of the sample fall within the measurable range of the steady-state method, it is often the better choice.

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Neither the steady-state method nor the transient method is inherently superior; each is suited to different types of materials and samples. For those new to these methods, the differences may be hard to grasp, but if you're unsure, please feel free to contact Bethel Hudson Laboratory. Both methods are available, and we’re happy to answer any questions!