Highpurity Alumina Fuels Nextgen Industrial Growth

Imagine a small material that can drive innovation in display technology, ignite hope for new energy solutions, upgrade the automotive industry, accelerate semiconductor breakthroughs, and even push computer performance to new heights. This is not science fiction but the reality being achieved by high-purity alumina.

Alumina: From Basic to Exceptional

Alumina (Al₂O₃), a seemingly ordinary material, plays critical roles across numerous industries due to its exceptional physical and chemical properties. It is heat-resistant, highly insulating, wear-resistant, and corrosion-resistant, making it ideal for manufacturing refractory materials, spark plugs, integrated circuit substrates, and more. However, when alumina reaches purity levels above 99.99% with uniform fine particles, it transforms from a basic material into a high-performance component, becoming essential for high-pressure sodium lamp tubes, sapphire watch surfaces, high-strength ceramic tools, and magnetic tape abrasives.

Soaring Demand: The Golden Age of High-Purity Alumina

In recent years, the rapid development of emerging industries such as display materials, energy, automotive, semiconductors, and computing has led to explosive growth in demand for high-purity alumina. To meet this market need, Sumitomo Chemical has successfully achieved large-scale production of high-purity alumina through its proprietary aluminum alkoxide hydrolysis process. Since establishing its first factory with an annual capacity of 250 tons in 1981, Sumitomo Chemical has continuously expanded production, reaching 1,500 tons per year by 2004. The company has also developed high-quality alumina powders tailored to various application requirements.

Technical Insight: The Science Behind Aluminum Alkoxide Hydrolysis

While several industrial methods exist for producing high-purity alumina—such as thermal decomposition of ammonium alum, thermal decomposition of aluminum ammonium carbonate (AACH), underwater spark discharge of aluminum, and vapor-phase oxidation—the aluminum alkoxide hydrolysis method stands out for its unique advantages. This process involves synthesizing high-purity aluminum alkoxide from aluminum metal and alcohol, hydrolyzing it to form hydrated alumina, and finally calcining it to obtain high-purity alumina.

Chemical Reaction Equations:

Al + 3ROH → Al(OR)₃ + 3/2H₂ (1)

2Al(OR)₃ + 4H₂O → Al₂O₃·H₂O + 6ROH (2)

Al₂O₃·H₂O → Al₂O₃ + H₂O (3)

The key to this method lies in the distillation purification of aluminum alkoxide and strict control of hydrolysis conditions to prevent the formation of hard agglomerates during drying. Since aluminum alkoxide hydrolyzes rapidly, it tends to produce fine hydrated alumina particles that easily form difficult-to-disperse agglomerates.

Phase Transformation Control: The Key to Precision Production

When hydrated alumina (such as boehmite) is calcined, it undergoes intermediate phases including γ, δ, and θ-Al₂O₃ before finally transforming into the high-temperature stable α-Al₂O₃. These intermediate-phase alumina particles are typically ultrafine, measuring just tens of nanometers. The transition from intermediate-phase alumina to α-Al₂O₃ requires temperatures above 1,200°C to rearrange the oxygen packing structure (cubic close packing/hexagonal close packing). The formation of α-phase nuclei is the rate-determining step in this transformation, and the nuclei density is relatively low. Once nuclei form, rapid grain growth occurs due to mass transfer from surrounding intermediate-phase alumina, resulting in micron-sized dendritic α-Al₂O₃ particles.

To obtain fine, uniformly sized α-Al₂O₃ particles, it is essential to maintain even temperature distribution during calcination, eliminate factors causing uneven nucleation, and complete the phase transformation at the lowest possible temperature. Research shows that the α-phase transformation temperature is significantly influenced by seed crystal addition, water vapor partial pressure in the calcination atmosphere, and elemental impurities. Adding α-Al₂O₃ seed crystals provides low-energy sites for nucleation and growth, while atmospheric water content enhances surface diffusion and accelerates grain growth in intermediate-phase alumina. Together, these factors reduce the activation energy of the α-phase transformation, thereby lowering the required transformation temperature.

Deagglomeration: Ensuring Optimal Performance

α-Al₂O₃ particles obtained through carefully controlled hydrolysis, drying, and calcination processes are typically agglomerated and require deagglomeration to achieve narrow particle size distributions. Various methods, including ball milling, vibration milling, jet milling, and wet media milling, can be employed for deagglomeration. In alumina ceramic applications, agglomerates can cause local inhomogeneities in green bodies and leave residual pores in sintered products. Particularly in transparent alumina ceramics for high-pressure sodium lamps, residual pores reduce light transmittance. For magnetic tape applications, agglomerates decrease surface smoothness and can damage magnetic heads during operation, impairing electromagnetic conversion characteristics. By refining processes to minimize agglomerates in high-purity alumina powders, Sumitomo Chemical has developed powders suitable for diverse applications.

With growing demand for highly functional submicron and nanoparticles, efficient jet milling and wet media milling technologies have advanced. Generally, powder deagglomeration must be handled carefully—smaller primary particles require greater attention to prevent reagglomeration and contamination. Through precise control of production conditions, Sumitomo Chemical has created high-purity alumina powders for various specialized uses.

Expanding Applications: The Boundless Potential of High-Purity Alumina

High-purity alumina finds wide-ranging applications that continue to expand with technological progress. Below, we highlight its roles in sapphire single crystals, plasma display panels (PDPs), automotive exhaust sensors, and semiconductor manufacturing.

1. Sapphire Single Crystals: The Foundation of LED Lighting

For decades, sapphire single crystals produced via the flame fusion method using γ-Al₂O₃ as a raw material have been valued for gemstones and watch surfaces due to their excellent properties. However, flame-fused sapphire suffers from poor crystallinity, limiting its applications. The edge-defined film-fed growth (EFG) method emerged to produce high-crystallinity sapphire with industrial scalability. EFG-grown sapphire is now widely used as substrates for high-brightness light-emitting diodes (LEDs) and support plates for polarizers in liquid crystal projectors. Particularly in high-brightness LEDs, white LEDs are expected to see broad adoption in advertising lighting, displays, automotive headlights, and home lighting—especially for mobile phone backlights, their current primary application.

LED devices are fabricated by growing GaN (a III-V compound) crystals on substrates. Sapphire serves as an ideal substrate due to its close lattice match with GaN and exceptional thermal stability at crystal growth temperatures. Sapphire starting materials must not only be high-purity but also minimize water absorption, as water can oxidize molybdenum crucibles during high-temperature melting above 2,000°C. Additionally, when continuously supplying α-Al₂O₃ to the process, particles must avoid fusing together to prevent equipment clogging. Spherical high-purity alumina AKQ-10 with ~2 mm particle size meets these requirements and is widely used as a sapphire starting material. Recent improvements in Czochralski single-crystal growth techniques have increased demands for higher packing densities in starting materials, enhancing industrial productivity. Responding to these needs, Sumitomo Chemical developed a new high-density α-Al₂O₃ for sapphire starting materials, achieving 2.0 g/cm³ packing density through optimized particle density and size distribution.

2. Plasma Display Panels (PDPs): The Future of Large-Screen Displays

Plasma display panels (PDPs) have gained attention as large, thin, flat-panel displays that enable slimmer and lighter devices. PDPs operate by exciting phosphors with vacuum ultraviolet (VUV) light at 147 nm (from Xe excimer radiation) and 172 nm (Xe resonance lines). Similarly, in cold-cathode fluorescent lamps used for LCD backlights, red, green, and blue phosphors are excited by 254 nm ultraviolet light from mercury atoms. Among these phosphors, the commercially used blue phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) is known to be the least stable. Heating during panel manufacturing and VUV exposure during PDP operation can degrade BAM's luminescence intensity and cause chromaticity shifts. Research continues to enhance brightness and improve degradation resistance.

Aluminate phosphors like BAM are typically manufactured by mixing high-purity alumina with Ba, Mg, and Eu compounds plus fluoride fluxes, then calcining—a solid-state reaction method. However, the process is complex, and phosphor manufacturers hold specialized knowledge. For example, while traditional fluoride-flux phosphors form square platelets with broad size distributions, Oshio et al. synthesized spherical aluminate phosphors matching the size and shape of non-fluxed alumina starting powders. These spherical phosphors match traditional products in chromaticity while offering 5% higher brightness and improved thermal stability. As aluminate phosphors become key materials for next-generation displays, demand is expected to grow. High-purity alumina plays a critical role in controlling phosphor characteristics, and Sumitomo Chemical continues developing tailored alumina powders for these applications.

3. Automotive Exhaust Sensors (A/F Sensors): Enabling Energy Efficiency

The market for air-fuel ratio (A/F) sensors—used to control engine combustion—is expanding rapidly. A/F sensors detect oxygen and residual unburned gas concentrations in exhaust to precisely regulate fuel injection. Proposed A/F sensor designs combine partially stabilized zirconia (an oxygen ion conductor) with alumina substrates (for electrical insulation and high thermal conductivity). To unitize these components, sintering must accommodate matching shrinkage rates and thermal expansion coefficients between materials. Minimizing thermal expansion differences is crucial to prevent interfacial cracking during operation. Additionally, both zirconia and alumina substrates require high density and fine grain size. Improving alumina's low-temperature sintering properties helps meet these demands. While smaller primary particles lower sintering initiation temperatures, they also reduce green density and may form hard agglomerates that impair sintered density. Sumitomo Chemical has developed various sinterable α-Al₂O₃ powders optimized for low-temperature sintering.

4. Semiconductor Manufacturing: Protection in Extreme Environments

Semiconductor and LCD manufacturing equipment extensively use α-Al₂O₃ components for superior plasma corrosion resistance. In sintered alumina bodies, reducing pores and impurities—while using Sumitomo Chemical's fine-particle Sumicorundum®—yields high-strength, corrosion-resistant ceramics free of residual porosity. Demand is also growing for plasma-sprayed alumina coatings on aluminum, nickel, chromium, zinc, zirconium, and their alloys. Semiconductor tool coatings require:

• High purity
• Good flowability for stable plasma flame feeding
• Particle shape retention before melting
• Complete melting during spraying

Single-crystal and large-particle α-Al₂O₃ Sumicorundum® meets these requirements, with demand expected to rise.

Nano Alumina: A New Era in Materials Science

Nanoscale α-Al₂O₃ represents a novel material poised to unlock new applications in abrasives, ceramics, and precision separation membranes.

(1) Abrasive Applications

Ultrafine α-Al₂O₃ leverages alumina's hardness for precision grinding and polishing. Sumitomo Chemical's HIT-series features edge-shaped particles for magnetic tape additives and metal/plastic abrasives. As tapes evolve toward thinner magnetic layers (<100 nm) and finer magnetic nanoparticles, incorporating α-Al₂O₃ nanoparticles becomes essential for wear resistance and head-cleaning performance. Research continues on nanoscale abrasives for chemical mechanical polishing (CMP).

(2) Ceramic Applications

Preventing nanoparticle agglomeration and minimizing green-body defects enables high-density sintering with fine grains. Ma et al. demonstrated 99% relative density and submicron grains by ball-milling nanoscale α-Al₂O₃ and sintering at 1,285°C. Sumitomo Chemical's wet-processed nanoscale alumina achieves 3.95 g/cm³ (99.2% density) at just 1,250°C.

(3) Separation Membrane Applications

α-Al₂O₃ porous membranes serve in ultrafiltration and gas separation due to chemical/thermal resistance. Hydrogen separation membranes integrated into steam reforming systems (CH₄ + H₂O → 3H₂ + CO) can lower reaction temperatures (800°C→500°C) while combining production and separation—key for future fuel cells. Membrane structures typically feature tubular α-Al₂O₃ supports with γ-Al₂O₃ intermediate layers topped by silica, zeolite, or palladium hydrogen-separation layers. However, steam promotes γ-Al₂O₃ grain growth/transformation, driving interest in α-Al₂O₃ alternatives. Studies show nanoscale α-Al₂O₃ slurries produce 40% porosity membranes with 10–60 nm pores, while α/γ-Al₂O₃ mixtures yield 2–50 nm pores. Finer primary particles enable smaller pore sizes (down to 16 nm), with applications expanding beyond gas separation to precision filtration.

As discussed, high-purity alumina—with controlled particle size, shape, and distribution—is a transformative material driving innovation in displays, energy, automotive, semiconductors, and computing. With rising demands, material producers must continuously enhance alumina powder performance. Particularly, nanoscale particle dispersion technology will be crucial for future breakthroughs. Moving forward, targeted process development and downstream integration will further expand alumina's remarkable potential.