High-purity alumina is crucial for enhancing the performance of a material within many different industrial processes. It features a controlled particle size distribution, tailored specific surface areas, and minimal impurities, making it ideal for applications requiring high thermal conductivity and electrical insulation properties. The powder’s minimal impurity content minimizes the risk of contamination and defects, thereby improving the overall quality and longevity of the end products. This makes high-purity alumina powder an indispensable material for cutting-edge technological applications where precision and reliability are paramount. In this blog, we will analyze why high-purity alumina has high thermal conductivity and applications where this property is useful.
What is High Purity Alumina?
Alumina or aluminum oxide (Al2O3) is a white crystalline powder that is derived from bauxite, a naturally occurring mineral. Standard-grade alumina is produced through the Bayer process. For applications that require higher purity, such as in LED substrate production (read: high purity alumina for LED substrates), semiconductor manufacturing, etching, or advanced ceramics (read: high purity alumina for technical ceramics), alumina undergoes additional refining to remove impurities, producing what is known as high-purity alumina (HPA). While alumina is widely used as a raw material in the production of aluminum metal, HPA stands out for its unique chemical properties and is useful in a range of high-tech and industrial applications.
Alumina (Al2O3) powder and alumina chemical structure
Basic Concepts of Thermal Conductivity
Thermal conductivity refers to a material’s ability to transfer heat. Different materials perform this task with varying levels of efficiency. A simple example of this is seen in cooking: if you leave a stainless steel spoon in a pot of boiling water, it becomes hot to the touch. However, if you leave a wooden spoon in the pot, the temperature of the wooden spoon will remain much cooler. The key difference lies in the way heat travels through these materials.
Heat Transfer Between Areas of Different Temperatures
When heat is applied to a material, it spreads from the hot areas, where there is high particle agitation, to the cooler areas, eventually bringing the material to thermal equilibrium. This means that the temperature throughout the material will become uniform. The concept can be viewed in the image below. Heat will travel from an area with a greater temperature (Temperature 2) to an area with a lesser Temperature 1.
So, why does the metal spoon heat up, but the wooden spoon does not? Several factors influence the thermal conductivity of a material, including its composition, physical properties, environmental conditions, and structural characteristics. Among these, one of the most important factors is the chemical structure or makeup of the material.
Heat Transfer in Different Materials
Heat transfer occurs differently across different material types. Metals generally have the highest thermal conductivity, primarily because they contain free electrons. In metals, the outer electrons of the atoms can move freely throughout the material. When heat is applied, energy travels through these free electrons, which gain energy and move quickly. This rapid movement allows the heat to spread from one electron to another, efficiently dispersing the heat throughout the metal. For example, silver has the highest thermal conductivity of any metal at 406 W/m·K.
Heat transfer in materials, showing conduction in metals (left) where thermal energy is transferred via free electrons, and in ceramics (right) where heat transfer occurs primarily through lattice vibrations due to the absence of free electrons.
In contrast, non-metallic solids, like high-purity alumina, do not conduct heat as effectively as metals. This is because non-metals lack free electrons. In these materials, the electrons are tightly bound to the atoms and cannot move freely. Instead, they transfer heat by vibrating, which is less efficient than the movement of free electrons. As a result, non-metals typically have low thermal conductivity, around 1 W/m·K. But there are some exceptions, like alumina that has thermal conductivity of around 25–35 W/m·K. We’ll explore later how its structure and high purity help it achieve this property.
| Temperature (K) | Conductivity (W/m·K) | Temperature (°C) | Conductivity (W/m·K) |
| 0 | 0 | 25 | 36 |
| 10 | 1,200 | 100 | 29 |
| 20 | 3,800 | 300 | 16 |
| 40 | 5,900 | 500 | 10 |
| 50 | 5,000 | 700 | 7.5 |
| 60 | 2,300 | 900 | 6.3 |
| 80 | 790 | 1,100 | 5.9 |
| 100 | 400 | 1,300 | 5.9 |
| 200 | 100 | 1,500 | 5.4 |
| 1,700 | 5.9 | ||
| 1,900 | 6.3 |
Relationship between the Atomic or Crystalline Structure, Microstructure, and Thermal Conductivity
From the modes of heat transfer described above, we can see that for alumina, thermal conductivity heavily depends on how well the vibrations can move through the it, and that depends on some physical properties and material structure.
Phase and Thermal Conductivity
From the model of heat transfer in ceramics discussed above, the easier the vibrations can propagate, the more efficient the heat transfer becomes, yielding higher thermal conductivity. One physical property that can affect this is the phase of the material.
Atomic arrangement in liquids and solids
In a solid, the atoms are closely packed together, making it easier for phonons to travel through the material. In liquids, however, the atoms are more loosely arranged, making heat transfer through vibration more difficult. In gases, the situation becomes even more challenging. The atoms or molecules are much farther apart, and the collisions between them are less frequent. Heat transfer is far less efficient, resulting in very low thermal conductivity in gases compared to solids and liquids.
General trend of increasing thermal conductivity from gases to liquids to solids. Among solids, metals typically exhibit higher thermal conductivity than nonmetals due to free electron transport.
Crystalline Structure, Morphology, Microstructure, and Thermal Conductivity
The crystal structure of a material can also influence thermal conductivity. In crystalline materials, atoms are arranged in a repeating, predictable pattern, like a well-ordered grid where each atom sits in a specific spot. This structure allows heat to travel in a predictable pattern, making crystalline materials more thermally conductive than amorphous materials, which have disordered atomic structures.
Crystal structure of alpha alumina (α-Al₂O₃), showing its hexagonal close-packed arrangement
Alumina can take different forms, each with its own atomic arrangement. Polar Performance Materials’ high-purity alumina, like HPA – SDF and HPA – SCD1, is in the α-Al2O3 phase, which has a hexagonal close-packed (hcp) arrangement. One of the main benefits of this structure is its high atomic packing factor (APF), which measures of how efficiently atoms fill space. The APF of the hcp structure of α-alumina is 0.74, meaning 74% of the space in the crystal is occupied by atoms. This is the highest possible packing efficiency for any atomic arrangement, not just in alumina, but in all crystalline materials.
When it comes to heat transfer and thermal conductivity, this tightly packed crystal structure allows thermal vibrations (phonons) to pass through the material quickly and evenly. In contrast, amorphous forms of alumina, where atoms are randomly arranged, or other phases with lower packing factors, cause have more scattering of these vibrations. This disorder leads to phonon scattering, which lowers the thermal conductivity of the material.
When we zoom out from the atomic scale, the next level of analysis is the particle morphology, which reveals how alumina takes shape as individual particles. Alumina can be rods, fibers, flakes, and spheres, each suited for different applications. For instance, fibrous nano-alumina possesses strong anti-sintering properties, making it an ideal additive for epoxy resin to enhance its tensile strength and rigidity. Flake-like alumina, on the other hand, is commonly used as a seed crystal in ceramics, significantly improving their toughness.
From Xie, Y., Kocaefe, D., Kocaefe, Y. et al. The Effect of Novel Synthetic Methods and Parameters Control on Morphology of Nano-alumina Particles. Nanoscale Res Lett 11, 259 (2016). https://doi.org/10.1186/s11671-016-1472-z
Polar Performance Materials’ high-purity alumina is available in either spherical or irregular particle forms. Spherical particles have higher flowability and packing density, making them ideal as fillers or components for battery separators, thermal interface materials (TIM), and advanced ceramics. Irregular particles, on the other hand, increase surface area and improve mechanical interlocking, making them good for coatings and catalyst applications.
| Product Name | HPA-SDF | HPA-SDC1 | HPA-M1 | HPA-M3 | HPA-M5 | HPA-M-HT | HPA-LU-M | HPA-4N- SMB | 4N-SMBJ | 4N-HT-SMB |
| Chemical Purity | 5N | 5N | 5N | 5N | 5N | 5N | 4N-5N | 4N | 4N | 4N |
| Crystalline Phase | α | α | α | α | α | α | α | α | α | α |
| Particle Shape | Spherical | Spherical | Irregular | Irregular | Irregular | Irregular | Irregular | Spherical | Irregular | Spherical |
| Specific Surface Area (m2/g) | 6-12 | 8-12 | 8-15 | 8-15 | 8-15 | 4-8 | 8-15 | 12-18 | 12-18 | 8-12 |
The morphology (particle size and shape) of Al₂O₃ can be influenced by several factors, including raw materials, concentrations, synthesis methods, additives, and heat treatment processes. These factors affect how the particles pack and densify during processes like sintering, which in turn influences the final microstructure of the material. Better packing and densification (less porosity) usually leads to higher thermal conductivity, while less efficient packing can reduce it.
| Calcination temperature (°C) | 600 | 800 | 1050 | 1200 |
| Alumina crystal type | Amorphous | γ-Al2O3 | α-Al2O3 | α-Al2O3 |
| Grain size (nm) | – | 5–50 | 10–100 | 100 |
| Color | Light yellow | White | White | White |
The microscope images below reveal that sintered ceramics from Polar Performance Materials’ high-purity alumina has a smaller grain size compared to the reference materials. Smaller grains mean there is a higher concentration of grain boundaries. A grain boundary is simply the boundary or “border” between two individual grains, or crystals, in a material. Imagine grains like little puzzle pieces that fit together. The boundaries are where these pieces meet. Grain boundaries can influence a material’s properties, like its strength or how it reacts to stress. Additionally, the microstructure of this high-purity alumina shows no gaps or voids, meaning it is very dense and uniform compared to the reference material. Therefore, the high packing ratio of this high-purity alumina result in better heat transfer performance, making it an ideal material for applications requiring efficient thermal management.
Scanning electron microscope (SEM) image of sintered high-purity alumina from Polar Performance Materials, showing a fine-grained, dense microstructure with no visible voids
Relationship between Purity and Thermal Conductivity
As mentioned earlier, alumina is naturally known for its high thermal conductivity due to its closely packed structure. Standard alumina is typically around 99.8% pure, but when it is refined to high-purity alumina, with a purity of 99.99%, its thermal conductivity is further increased. But, how exactly do impurities affect thermal conductivity?
Impurities and defects in alumina can disturb the smooth flow of thermal vibrations through the crystal lattice. These disruptions act like obstacles, scattering the vibrations and decreasing the material’s thermal conductivity. Impurities in materials can disrupt the crystal lattice, weakening its ability to conduct heat effectively. This is especially problematic in high-performance industries like electronics, aerospace, and e-mobility, where effective heat dissipation is critical.
One common impurity that significantly affects thermal conductivity is sodium. As an alkali metal, sodium introduces distortions in the crystal structure, which impede heat flow. While traditional alumina may contain sodium levels above 100 ppm, Polar Performance Materials’ high purity alumina produced through an efficient and a sustainable process maintains Na levels below 10 ppm. Thanks to its low impurity content and uniform structure, high purity alumina enables better phonon transport and offers superior thermal performance.
Elemental Composition of Polar Performance Materials’ High-Purity Alumina (Measured by GDMS)
Applications of High Purity Alumina requiring High Thermal Conductivity
Thermal conductivity is an important factor when selecting materials for designs where heat transfer is essential. For example, when designing a temperature sensor for an airplane, the material must be able to handle rapid temperature changes as the aircraft ascends and descends, potentially reaching temperatures as low as –76 °F. In this case, a material like High Purity Alumina Technical Ceramic would be ideal. Its high purity enhances its conductivity, allowing the material to quickly respond to temperature changes without degrading. This makes it perfect for high-performance applications where quick heat transfer is required and the material must endure extreme environmental conditions without damage.
High-purity alumina is also widely used in the semiconductor industry. One of its key applications is in the production of sapphire, a crystalline form of alumina, which is used as a wafer substrate for light emitting diodes. Effective heat management is important in semiconductor devices to prevent overheating, which can lead to degradation and malfunction.
High Purity Alumina available at CAPLINQ
If you’re in need of high-purity alumina for advanced semiconductor manufacturing, cutting-edge electronics, next-generation lithium-ion batteries, or high-quality sapphire production, discover how Polar Performance Materials’ high-purity alumina can enhance your products with superior thermal conductivity. Contact us today to learn more about our offerings or to request a sample for your next high-performance application and work together to advance sustainability in your industry.
