The Role of Crucible Materials in Thermal Analysis Techniques

In thermal analysis techniques such as Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Simultaneous Thermal Analysis (STA, a.k.a TGA/DSC or TGA-DSC), the choice of crucible material plays a vital role in determining the accuracy, reliability, and reproducibility of results.

Crucibles are pans that hold the sample during thermal experiments, and their interaction with both the sample and the heat source can significantly impact the outcome of the analysis. Factors such as thermal stability, chemical reactivity, and heat transfer properties of the crucible material must be considered carefully.

Crucibles on a table

Impact of Crucible Materials on Thermal Analysis

In thermal analysis, the choice of crucible material directly affects the accuracy and reliability of the results.

In TGA, where the mass of a sample is measured as it is heated, the crucible material plays a key role in ensuring accurate and precise weight loss measurements.

• Thermal stability: Crucibles used in TGA must be able to withstand high temperatures without deforming or degrading. If the crucible loses its integrity during analysis, it can lead to inaccurate weight measurements and thermal behavior misinterpretation. Selecting a crucible material with high heat resistance is critical, especially when analyzing materials at extreme temperatures.
• Chemical reactivity: TGA experiments often involve samples that decompose, oxidize, or react during heating. The crucible material should be inert to avoid reactions with the sample that could alter the measurement. Chemical reactivity can also lead to contamination of the sample or degradation of the crucible, affecting the experiment's repeatability.
• Crucible mass: The mass of the crucible influences the sensitivity of TGA measurements. Heavier crucibles can reduce the precision of mass change detection, particularly for small samples. Lighter crucibles are preferred for more sensitive analyses, where even minor weight changes need to be accurately detected.

In DSC, the selection of crucible material affects heat flow measurements and the ability to detect heat flow changes during phase transitions, melting, and other thermal events.

• Thermal conductivity: This is the most important factor for DSC. Crucibles with high thermal conductivity enable efficient heat transfer between the sample and the sensor, which is critical for detecting subtle thermal events. Low conductivity can result in delayed heat flow measurements, making it difficult to pinpoint exact transition temperatures.
• Temperature tolerance: The crucible must withstand the temperature range of the DSC experiment without melting or degrading. A material with insufficient temperature tolerance can lead to thermal decomposition or a failure to capture important events at high temperatures, potentially compromising the experiment.
• Crucible mass: Similar to TGA, the mass of the crucible plays a role in DSC as well. Heavier crucibles may delay heat flow detection, especially for fast or sharp transitions. Lighter crucibles are ideal for improving responsiveness and ensuring accurate heat flow measurements, particularly in sensitive applications like pharmaceutical or polymer analysis.

In STA, which combines TGA and DSC in one measurement, crucible factors such as heat resistance, reactivity, and conductivity become doubly important. Materials should accommodate both precise mass loss measurement (TGA) and accurate heat flow detection (DSC), making the selection process more stringent.

The most commonly used crucible materials in thermal analysis include platinum, aluminum, alumina (Al2O3), stainless steel, and copper, with each offering distinct properties that influence the performance of the analysis.

• Platinum crucibles: Renowned for their exceptional chemical inertness and thermal stability, platinum crucibles are ideal for high-temperature applications. They can withstand temperatures exceeding 1000°C and resist oxidation and corrosion, making them suitable for reactive or high-temperature samples. Platinum crucibles also show better curves than alumina crucibles due to their better thermal conductivity. However, molten metal can easily alloy with platinum.
• Aluminum crucibles: Lightweight and cost-effective, aluminum crucibles are the standard DSC crucibles with suitability for TGA applications as well, particularly those involving lower temperatures. They offer excellent thermal conductivity, which allows for rapid heating and cooling, enhancing the resolution of thermal events. However, aluminum melts at 660 ℃, thus not suitable for high-temperature applications, and they may react with acidic or basic samples.
• Alumina (Al2O3) crucibles: alumina crucibles are the most common crucibles for TGA. They are highly resistant to high temperatures and corrosive environments, especially in extreme (>1600 ℃) temperatures. They are also suitable for STA with suppressed heat flow signal. However, they are not recommended for DSC applications due to their low thermal conductivity, which can interfere with the precision of heat flow measurements in DSC.
• Stainless steel crucibles: Stainless steel crucibles are suitable for analyzing materials in pressurized environments due to their ability to withstand both high temperatures and pressure. They are heavier than other crucibles, lowering temperature gradient. A layer of gold can be coated to prevent unnecessary reactions.
• Copper crucibles: Copper crucibles are used primarily in oxidation stability or induction time tests due to their excellent heat conduction properties. However, their reactivity with many chemicals limits their use to specific analyses where oxidation behavior is of prime interest.

Durability and reusability are crucial when selecting crucibles, particularly in laboratories that conduct frequent thermal analyses.

• Platinum crucibles: Known for their long-term durability, platinum crucibles can endure repeated high-temperature cycles and aggressive chemical environments without degrading. Their ability to be reused multiple times makes them cost-effective despite their high initial price.
• Aluminum crucibles: While affordable, aluminum crucibles are typically designed for single-use or limited reuse due to their susceptibility to deformation and chemical reactions at high temperatures.
• Alumina crucibles: alumina crucibles strike a balance between cost and durability, with the ability to withstand high temperatures and resist chemical interactions. Since maintenance for alumina crucibles is easy, they can be reused multiple times with proper care.
• Stainless steel crucibles: Commonly used in high pressure circumstances, stainless steel crucibles are highly durable. They can be used repeatedly.
• Copper crucibles: Copper is highly oxidizable, and their reusability is also limited only to specific oxidation-related experiments.

When selecting the appropriate crucible material for thermal analysis, key concerns laboratories may face besides durability and reusability include chemical reactivity, temperature resistance, accuracy, and cost-effectiveness.

• Chemical reactivity: The potential for chemical reactions between the sample and the crucible material is a major concern in thermal analysis. If the crucible material reacts with the sample, it can alter the thermal properties being measured, leading to false conclusions.
• Temperature resistance: The crucibles used must resist the temperature of the analysis and do not introduce foreign substances during heating. For example, platinum and alumina crucibles are used in extreme temperature conditions, while aluminum crucibles are used in moderate temperature (≤600 ℃) conditions.
• Accuracy of results is another critical consideration, particularly when analyzing subtle or small thermal events. For instance, the thermal conductivity of the crucible material can influence the resolution of phase transitions or mass changes in DSC, thus the crucible should conduct heat not only well but also uniformly.
• Cost-effectiveness is also unavoidable when selecting crucible materials. Platinum crucibles often cost high, but their reusability compensates the price. Aluminum crucibles, while being cheap, perform poorly in effectiveness due to their limited reusability. Laboratories should take their budget into consideration when making decisions.

The choice of crucible material is a critical factor in ensuring the accuracy and reliability of thermal experiments. Each material—platinum, aluminum, alumina, stainless steel, and copper—offers distinct advantages based on the thermal requirements and sample characteristics of the analysis.

By carefully selecting crucibles based on the specific needs of their analyses, researchers can ensure precise, reproducible, and efficient thermal analyses, ultimately improving the quality of their data and the cost-effectiveness of their operations.

For further questions regarding thermal analysis crucibles and inquiries, feel free to contact WelchLab or purchase our crucibles online, and we will accompany you in your laboratory success.

Appendix: Table of crucible materials and their properties