Fillers are materials added to polymers, composites, and other substances to modify properties, reduce costs, or fill spaces. In niche advanced materials, such as epoxy molding compounds and thermal interface materials, fillers increase mechanical strength, thermal conductivity, and electrical insulation. They also help control thermal expansion and improve processing characteristics.
Two of the most common fillers are silica (SiO2) and alumina (Al2O3). Each offers distinct advantages and limitations. Silica, with its lower density of 2.65 g/cm³ (compared to alumina’s density of 3.99 g/cm³), is often preferred for applications requiring lightweight and good insulation properties, like paints, coatings, and rubbers. However, silica’s lower mechanical strength and thermal conductivity make it less suitable for high-performance applications. This is unlike alumina, which provides superior mechanical strength, thermal conductivity, and abrasion resistance, making it ideal for demanding environments like advanced ceramics, thermal interface materials, and electronics. Additionally, due to its higher density, alumina is better suited for applications that require enhanced durability and performance under high stress and temperature. When the purity of alumina exceeds 99%, as in high purity alumina (HPA), these advantages are significantly enhanced, making it an ideal filler for materials used in demanding, high-performance applications, like aerospace, electronics, and automotives.
Comparison of Silica and Alumina Filler Properties
| Property | Silica | Alumina |
|---|---|---|
| Thermal Conductivity | Low (~1.3 W/m·K) 🟥🟥 | Very High (15–35 W/m·K) 🟩🟩🟩🟩🟩 |
| Density | Low (2.2 g/cm³) 🟥🟥 | High (3.9 g/cm³) 🟩🟩🟩🟩 |
| Hardness | Moderate 🟨🟨🟨 | Very High 🟩🟩🟩🟩🟩 |
| Thermal Stability | High 🟩🟩🟩🟩 | Very High 🟩🟩🟩🟩🟩 |
| Electrical Insulation | Excellent 🟩🟩🟩🟩🟩 | Excellent 🟩🟩🟩🟩🟩 |
| Cost | Low 🟩🟩🟩🟩 | High 🟥🟥 |
| Preferred For | Lightweight, electrically insulating, and cost-effective fillers, especially where low thermal conductivity is acceptable or even preferred | High thermal conductivity, abrasion resistance, and better heat dissipation, especially in power electronics and LED applications |
In this blog, we will identify the properties of HPA that make it an ideal filler material. Then, we will discuss how HPA is used as a filler in advanced materials, like epoxy molding compounds, thermal interface materials, and battery separators.
Properties of High Purity Alumina that Make it an Ideal Filler
High Purity
To start with, why high purity alumina is an ideal filler, we can begin with the first two words in its own name, high purity. These two words are what differentiate this compound from basic alumina and what gives traditional alumina properties the boost they need to be used for highly demanding applications. High purity alumina with ≥99.99% purity contains extremely low levels of impurities. This is important because impurities can cause unwanted phases to form, which affects alumina’s quality and performance. For example, calcium and iron impurities can form calcium, which can degrade alumina’s properties. In applications, calcium impurities have been shown to cause a calcium-rich phase (Ca₃Co₄O₉) to separate in all-solid-state sodium-ion batteries. This phase separation leads to both performance loss and reduced battery stability.
Additionally, HPA has very low amounts of silica. During the production of traditional alumina, trace amounts of silica can be introduced, especially if the bauxite ore contains a lot of silica. This affects the final alumina because instead of having a pure material, you get a mix of Al₂O₃ and SiO₂.
Lastly, caustic soda (Na₂O), which is one of the main impurities in alumina, has an undesirable effect on its properties in certain applications. In alumina ceramics, it reduces electrical insulation and increases loss tangent, which quantifies the energy lost as heat when an electric signal passes through the material. A lower loss tangent means the signal travels more easily.
Polar Performance Materials’ efficient (only 7 steps vs. 21 in traditional processing) and sustainable high-purity alumina manufacturing process produces alumina with significantly lower soda content and ultra-low impurity levels (Table 1).
| Purity | Impurity Concentration [ppm] | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Na | Mg | Si | Ca | Ti | Cu | Cr | Fe | K | |
| 3N (99.9%) | <150 | <20 | <50 | <50 | <20 | <10 | <5 | <50 | <20 |
| 4N (99.99%) | <30 | <5 | <10 | <10 | <5 | <5 | <5 | <20 | <10 |
| 5N (99.999%) | <4 | <1 | <3 | <3 | <0.5 | <0.5 | <1 | <1 | <0.5 |
Tightly-packed Crystalline Structure and Microstructure
At the atomic level, alumina is made of aluminum and oxygen atoms held together by strong ionic bonds. These bonds create a highly organized, repeating pattern. You can imagine a neatly arranged brick wall, but with atoms instead of bricks.
Alumina can take different forms, each with its own unique atomic arrangement. Polar Performance Materials’ high-purity alumina is in the α-Al2O3 phase, a hexagonal close-packed (hcp) structure. In this structure, aluminum ions are surrounded by oxygen ions in a layered pattern, making it the most thermodynamically stable form of alumina.
One key advantage of this structure is its high atomic packing factor (APF), which measures how efficiently atoms fill the available space in a crystal. The hcp structure of α-alumina has one of the highest APFs (0.74), meaning it packs atoms more efficiently than other structures like body-centered cubic (BCC) at 0.68 or simple cubic (SC) at 0.52. This crystalline structure has multiple benefits, one of them being increased thermal conductivity. Due to the packed structure, heat vibrates evenly throughout the atoms, making it easier for the material to handle heat effectively.
If we observe alumina at the microscopic level, we can see its microstructural features, including grains, grain boundaries, and other fine details. The microstructure shows how different phases and defects are arranged within a material, typically on a nm to µm scale.
The microscope images below reveal that 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.
Scanning electron microscope (SEM) image of high-purity alumina from Polar Performance Materials, showing a fine-grained, dense microstructure with no visible voids
High Purity Alumina particles are engineered to be of uniform size, and this characteristic is crucial for fillers, for which we want the base material to blend evenly into the filler. The very small particle sizes help enhance flowability, which in turn helps with the processing of the material for which HPA has been used as a filler.
High Thermal Conductivity
Thermal conductivity refers to a material’s ability to transfer heat efficiently. This property is influenced by several factors, including molecular structure, crystalline structure, electron movement, and impurities. While metals conduct heat well through the movement of free electrons, non-metals, like HPA, rely on phonons (vibrations in the atomic lattice) for heat conduction. As discussed above, the dense packing in the crystalline structure of HPA enhances phonon movement, improving its thermal conductivity.
Additionally, impurities can disrupt the atomic structure and scatter phonons, reducing thermal conductivity, but HPA’s ≥99.99% purity ensures minimal impurities, promoting efficient heat transfer. Learn more about the thermal conductivity of high purity alumina in our blog, Why does High Purity Alumina have High Thermal Conductivity?
High Mechanical Properties
The high hardness of HPA makes it an exceptionally strong material, ranking 9 on the Mohs scale as one of the hardest known materials. This extreme hardness is crucial for applications requiring resistance to mechanical stress, ensuring durability in demanding environments. Unlike metals, which can weaken under high temperatures, alumina retains its structural integrity even after cooling, avoiding the degradation commonly seen in metals. Its hardness also directly contributes to superior abrasion resistance, making it ideal for applications involving constant friction, impact, or exposure to abrasive materials. Additionally, the strength and hardness of alumina further improve with higher purity levels. High-purity alumina (≥99.99%) exhibits significantly enhanced mechanical properties, making it highly valued in industries such as aerospace, electronics, and industrial machinery, where materials must withstand the most demanding conditions.
Vickers hardness (HV) vs. Mohs hardness of the 10 hardest materials
Excellent Chemical Stability
One of the most advantageous properties of high-purity alumina is its good chemical resistance. This comes from the strong ionic bonds, with a certain degree of covalent bonding, between aluminum and oxygen atoms, which are difficult to break. As a fully oxidized and highly stable ceramic, HPA resists further oxidation and does not corrode easily, which is important for applications that need long-term stability.
HPA is chemically inert. This means HPA ceramics do not react with most acids, alkalis, or other corrosive substances, even at elevated temperatures. Its high purity, low porosity, and stable crystalline structure limit the diffusion of impurities and enhance its resistance to harsh chemical environments.
Ionic bonding in aluminum oxide (Al₂O₃)
High Purity Alumina as Fillers in Thermal Interface Materials (TIMs)
What are Thermal Interface Materials?
Thermal Interface Materials (TIMs) are specialized compounds designed to enhance thermal conductivity between heat-generating components and heat sinks or spreaders in electronic devices. By minimizing interfacial thermal resistance, TIMs play a critical role in maintaining optimal operating temperatures and ensuring device performance, reliability, and longevity.
Typically, TIMs are composed of a polymer matrix such as silicone, epoxy, or urethane infused with thermally conductive fillers. These fillers, which include materials like alumina, boron nitride, and metal oxides, are essential for the material’s heat transfer efficiency.
Functions of HPA Fillers in TIMs
When incorporated into a polymer matrix, HPA particles help form continuous thermal conduction pathways through which heat can efficiently travel from one surface to another, typically from a heat-generating component to a heat sink.
High-purity alumina powder and its dispersion in thermal interface material (TIM), illustrating how the alumina particles form conductive pathways that enhance heat transport efficiency
To maximize this effect, the HPA particles must be densely packed and well-dispersed within the matrix. Gaps or voids between particles introduce thermal resistance, disrupting the continuity of the heat conduction path and reducing overall thermal performance. Therefore, the particle size, shape, and distribution of the alumina filler are critical factors. Spherical or engineered particle geometries can help improve packing density, while narrow particle size distributions reduce void formation and promote better inter-particle contact.
High-purity alumina is particularly preferred due to its consistent thermal properties, chemical inertness, and low impurity levels, which minimize the risk of degradation or unwanted reactions over time. These characteristics make it an ideal filler material for high-performance TIMs used in electronics, power devices, and automotive applications where heat dissipation is critical for long-term reliability.
High Purity Alumina as Fillers in Epoxy Molding Compounds
What are Epoxy Molding Compounds?
An epoxy molding compound (EMC) is a thermosetting polymer material widely used in the encapsulation of electronic components, such as semiconductors, to provide mechanical support, environmental protection, and enhanced thermal and electrical performance. EMCs are essential in ensuring the reliability and longevity of electronic devices by shielding them from physical damage, moisture, and temperature fluctuations.
Epoxy molding compound (EMC) pellets and their use in a semiconductor package. The EMC protects the chip and other components by sealing and insulating the device.
Importance of High Purity Alumina Fillers in Epoxy Molding Compounds
EMCs typically contain an epoxy resin as the base matrix, curing agents to facilitate cross-linking, and a range of fillers to enhance their mechanical, thermal, and electrical properties. Common fillers include silica, alumina, and other inorganic materials that contribute to the compound’s rigidity, thermal conductivity, and dimensional stability.
High Purity Alumina is gaining distinction as a common filler in EMC formulations, particularly for applications requiring superior thermal management and reliability. HPA’s excellent thermal conductivity helps dissipate heat efficiently from encapsulated components, preventing overheating and ensuring stable operation. Its fine particle size ensures uniform dispersion within the epoxy matrix, reducing internal stresses and improving the material’s overall performance. Additionally, HPA’s high purity minimizes the risk of ionic contamination, making it especially valuable in advanced electronics, such as power modules and automotive control units. By integrating HPA, EMCs achieve enhanced thermal, mechanical, and electrical properties, supporting the demands of modern electronic technologies. EMCs require low radioactive elements in their composition because radioactive elements can cause failures in electronic devices. Polar Performance Materials’ HPA-LU-M is low in both Uranium and Thorium (Th), making it ideal for use as a semiconductor epoxy molding compound filler.
Polar Performance Materials’ HPA-LU-M
High Purity Alumina Filler for Semiconductor Epoxy Molding Compounds
Purity Range: 4N to 5N (99.99% to 99.999%)
Low Radioactive Impurities: Extremely low uranium and thorium levels
Particle Morphology: Irregular shape for enhanced surface contact and dispersion
Specific Surface Area: 8 to 15 m²/g
Particle Size Distribution (D50): 1-5 µm
Applications: Semiconductor EMC fillers, advanced ceramics, specialty coatings, and optoelectronics
High Purity Alumina isn’t just another filler. It’s a performance booster! Its unique combination of high thermal conductivity, strength, purity, and chemical resistance makes it a go-to choice for advanced materials in electronics, automotive, aerospace, and energy applications. Discover the benefits of using high-purity alumina for your applications. Contact us today!
