Unveiling the Thermal Properties of Steatite and Bisque Fired Alumina: An Insightful Experiment with the TLS-100

Understanding the thermal properties of materials is crucial in various industrial applications, particularly when dealing with high temperatures. This article delves into an experiment conducted in our lab utilizing the TLS-100, a portable and precise thermal conductivity testing device, to analyze the thermal conductivity and resistivity of two intriguing materials: steatite and bisque-fired alumina.

These materials are renowned for their heat-resistant properties and insulating nature, making them valuable assets in diverse industrial sectors. By delving deeper into their thermal behaviour, we gain valuable insights for optimizing their use in various applications.

What is Steatite?

Steatite, or soapstone, is a naturally occurring mineral composed primarily of talc, magnesium silicate, and minor amounts of other minerals. Its unique composition grants it exceptional properties, including:

• High Heat Resistance: Steatite exhibits excellent resistance to high temperatures, making it suitable for kiln linings, stove parts, and laboratory benchtops.
• Electrical Insulator: It possesses excellent electrical insulating properties, finding applications in electrical components and high-voltage equipment.
• Machinability: Steatite is relatively soft and easily machined, allowing for the creation of intricate shapes for various applications.

What is Bisque Fired Alumina?

Bisque-fired alumina is a specific form of aluminum oxide (Al2O3) that has undergone a partial sintering process. This process involves:

1. Pressing: Aluminum oxide powder, along with some additives, is pressed into the desired shape.
2. Partial Firing: The pressed shape undergoes a partial firing, creating small, weak bonds between adjacent alumina particles.

This partial sintering grants bisque-fired alumina several key characteristics:

• Machinability: The weak bonds allow for machining using conventional tools, making it ideal for creating complex shapes before final sintering.
• Malleable: The partially sintered state enables easier shaping and forming than fully dense alumina.
• High-Temperature Resistance: Bisque-fired alumina retains its inherent heat resistance even in its partially sintered state, making it suitable for various high-temperature applications.

Once you achieve your desired shape, the bisque-fired alumina undergoes a final sintering process, fully densifying the material and significantly increasing its strength and hardness.

This final product is widely used in:

• Electronics: Substrates for electronic circuits due to its excellent electrical and thermal insulating properties.
• Aerospace: Components requiring high thermal and mechanical resistance in demanding environments.
• Medical Devices: Applications requiring biocompatibility, high strength, and resistance to wear and tear.

The Role of the TLS-100 in Thermal Conductivity Testing

The TLS-100 is a portable and versatile thermal conductivity testing system. It utilizes the Transient Line Source (TLS) method, a well-established technique for measuring thermal conductivity and resistivity in various materials. This allows for convenient testing in the lab and in the field. The system performs measurements in accordance with ASTM D5334 —Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure.

This method offers several advantages:

• Portability: The TLS-100 is compact and lightweight, allowing on-site testing in diverse environments.
• Accuracy: The system adheres to the ASTM D5334 standard, ensuring reliable and accurate thermal conductivity measurements.
• Versatility: The TLS-100 can test a wide range of materials, including solids, liquids, pastes, and powders.

The Experiment Setup

The experiment measured the thermal conductivity of steatite and bisque-fired alumina samples using the TLS-100. The setup comprised the following:

• Attached Needle Probe: The TLS-100 was equipped with a specialized needle probe specifically designed for testing solid materials.
• Thermal Paste: A thin layer of thermal paste was applied to ensure optimal contact between the probe and the samples.
• Sample Sandwiching: Two pieces of each material (steatite and bisque-fired alumina) were placed on either side of the needle probe, creating a “sandwich” configuration.

Figure 1. The diagram on the left illustrates the method used to ensure excellent thermal contact between the needle probe of the TLS-100 and the material samples. The photo on the right depicts the two samples of bisque fired alumina during testing.

The TLS-100 completes measurements using the attached needle probe, which acts as the heating element and temperature sensor. Thermal paste is often used with solid samples to achieve maximum contact between the sensor and the sample.

Here’s a breakdown of the specific steps followed:

1. Sample Preparation: Two pieces of each material were obtained for the testing procedure.
2. Thermal Paste Application: The thermal needle was coated with thermal paste to ensure optimal contact with the samples.
3. Sample Sandwiching: The prepared needle probe was sandwiched between the two pieces of each material, as shown in Figure 1.
4. Temperature Stabilization: Before beginning measurements, a 15-minute wait period was observed to ensure the sample and sensor were isothermally stable.
5. Measurement Schedule: Five tests, each 120 seconds long, were conducted, spaced 15 minutes apart, to allow for temperature stabilization between measurements.

The Results and Implications

The experiment’s results revealed remarkable consistency across all five tests for steatite and bisque-fired alumina, highlighting the precision and repeatability of the TLS-100 system.

The mean thermal conductivity values obtained were:

• Steatite: 3.107 W/mK
• Bisque-Fired Alumina: 5.077 W/mK

Bisque-fired Alumina Results

Table 1. Thermal conductivity and thermal resistivity data collected by the TLS-100 on bisque-fired alumina.

Steatite Results

Table 2. Thermal conductivity and thermal resistivity data collected by the TLS-100 on steatite.

These values closely align with the accepted reference thermal conductivity values of 3 W/mK for steatite and 5-5.25 W/mK for bisque-fired alumina, validating the accuracy of the TLS-100 measurements. These findings hold significant implications for the industrial applications of steatite and bisque-fired alumina:

• Heat Transfer Management: Both materials exhibit relatively low thermal conductivity, making them valuable for applications requiring thermal insulation, such as furnace linings, heat shields, and electronic component housings.
• Temperature Control: Their ability to resist heat transfer makes them suitable for applications where maintaining consistent temperatures is crucial, such as laboratory equipment and medical devices.

The results produced by the TLS-100 showcase the low thermal conductivity of each material, which makes them such effective insulators. The close agreement between the experimental and reference values demonstrates the excellent accuracy of the TLS system, which makes it an ideal choice for thermal conductivity testing.

Conclusion

This study successfully employed the TLS-100 to analyze the thermal conductivity of steatite and bisque-fired alumina. The results provided valuable insights into their thermal behaviour, confirming their suitability for various industrial applications requiring heat resistance and thermal insulation. Further research exploring the thermal conductivity of these materials at varying temperatures and under different processing conditions could offer even more comprehensive data for optimizing their use in diverse industrial settings.

Frequently Asked Questions

What is the TLS-100, and how does it work in thermal conductivity testing?

The TLS-100 is a portable thermal conductivity testing system that utilizes the Transient Line Source (TLS) method. A small needle probe is inserted into the material, and a short heat pulse is applied. The TLS-100 calculates the material’s thermal conductivity and resistivity by monitoring the temperature change over time.

How was the experiment set up for the thermal conductivity testing of steatite and bisque-fired alumina?

The experiment involved:

1. Attaching a needle probe to the TLS-100.
2. Applying thermal paste to the samples.
3. Sandwiching the probe between two pieces of each material (steatite and bisque-fired alumina).
4. Five tests of 120 seconds each were conducted with a 15-minute interval between each test.

What were the experiment’s results, and what do they imply about the thermal properties of steatite and bisque-fired alumina?

The experiment yielded consistent results for both materials, with steatite having a mean thermal conductivity of 3.107 W/mK and bisque-fired alumina having a mean thermal conductivity of 5.077 W/mK. These values confirm their low thermal conductivity, making them suitable for heat insulation and temperature control applications.