August 02nd, 2019
Aluminum oxide (Al2O3), also known as alumina, is derived from a natural ore called bauxite. Natural Al2O3 occurs as the mineral corundum and in its gemstone forms as sapphire and ruby; however, the term alumina usually refers to the manufactured material (1).
The Bayer process was first developed in 1888, and used to extract most alumina from bauxite (1). Crushed bauxite is mixed with sodium hydroxide solution and seeded with crystals to precipitate aluminum hydroxide. A kiln is used to heat the hydroxide, in order to remove water. Alumina goes through several transition phases (α, χ, η, δ, κ, θ, γ, ρ) as the temperature increases. The most thermodynamically stable form reached at the highest temperatures is the alpha (α) phase. With its closely packed, hexagonal crystals, this phase is the strongest.
Alumina is extremely hard and strong, resists abrasion and chemicals, and has a high melting point (1). Other properties include high thermal conductivity, a high degree of refractoriness, high dielectric strength and high electrical resistivity (even at elevated temperatures).
The properties of alumina make it extremely useful for a wide range of industrial applications such as ceramic processing, metallurgy and chemical processing (1). It is an excellent ceramic electronic substrate and finds applications in abrasives, milling media, spark plugs, and wear-resistant applications such as pump seals and welding nozzles.
In this blog post, Saint-Gobain Surface Conditioning will detail the different grades of alumina, the history of α-alumina, monocrystalline alumina, micro aluminum oxide, alumina zirconia abrasive grains, and nano polycrystalline alumina particles
Making alumina using the Bayer process produces several grades of material. When temperatures reach higher than 1100ºC, α-alumina occurs as various ‘calcined’ grades with a range of crystal size, morphology and chemical impurities (2). The main impurity is sodium oxide (soda) and the grades are divided depending on how much soda they contain: for example, ‘medium soda’ contains 0.15-0.25% by weight and ‘low soda’ less than 0.1%.
Other calcined grades include ‘reactive’ alumina, which has a small crystal size and a relatively high purity (2). It is very strong and highly resistant to wear and temperature. ‘Fused’ alumina is made in electric arc furnaces. With its low porosity, low permeability, high density and high refractoriness, it is highly suitable for refractories and abrasives.
Saint-Gobain produces a variety of alumina types for many applications, but its particular specialty is abrasive applications (3). The inherent properties make it suitable for a wide variety of abrasive uses, both fixed and loose, particularly in many chemical mechanical polishing (CMP) applications.
In the 1990s, manufacturing processes produced loose abrasive alpha alumina (about 100-200nm) but there was little control over defects and morphology. During 2005-2015, technology improvements resulted in much more control. In recent years, these advances have progressed even further, and most recently developers are working towards sub-100nm particles with minimum defects and extreme chemical resistance.
Saint-Gobain invented α-alumina seeded gel technology in 1984 for the fixed abrasives market (4). In this ground-breaking technology, minute particles are dispersed in a precursor gel, then water is removed before it is converted to the alpha phase at relatively low temperatures. The result is a grain with a unique nano-structure consisting of sub-micron, highly uniform crystals. When stressed, microfracture results in new cutting edges, making them ideal for use as abrasives.
With Saint-Gobain’s seeded gel-based alumina particle platforms, it’s possible to control all aspects of the alumina particles: size, shape, size distribution, phase, doping, and mechanical and thermal properties.
Saint-Gobain has significantly improved its monocrystalline alumina (5). One environmental advantage is that sulfur is no longer used to make its MA88® product which is produced by fusing very pure alumina with special alloys. Its unique microstructure - each particle is a single crystal – create many edges, and these edges are sharp and tough. They confer a sharper cutting action when used as an abrasive, as well as making internal bonds stronger. Removal rates have improved by nearly 30% and the process requires less energy, too.
Applications include precision grinding wheels in the automotive and aerospace industries, and in rough grinding applications such as thin wheels and cut-off wheels.
The company’s portfolio of white fused alumina products extends from standard macro grits to specialized micro grits with purity levels around 99.5% aluminum oxide. This versatile range can be used as lapping compounds, micro-blasting, heat treatment setting sands, wear-resistant coatings and bonded abrasives.
Another range of products is Saint-Gobain’s micro aluminum oxides (6). Control over particle size and chemistry mean they can be adapted to suit specific applications for the industrial, semiconductor and electronics industries. The range includes white fused and brown fused aluminium oxide, calcined alumina with microgrit sizes ranging from 45 microns to 3 microns and electronic grade alumina.
One of the toughest abrasive materials is alumina zirconia (7). By monitoring the crystal size and microstructure, these abrasive grains can be manufactured to produce the optimum alloy of alumina and zirconia. It is their very fine crystalline microstructure that gives alumina zirconia the edge over other commodity abrasives such as silicon carbide. It performs well in coated abrasive, bonded abrasive, and cut-off wheel applications, in particular for stainless steel and other challenging alloys.
Over the years, Saint-Gobain has developed and refined its seeded gel technology. Its current Polycrystalline Alumina Particles now have a finer crystallite size than ever before at less than 75 nanometers. Particle dispersion has been enhanced, and particle size distribution can be closely controlled. What’s more, the product can be adapted for specialized applications.
The company’s selection of materials means that customers can choose from different sizes tailored to their application. For example, products can be made available in sizes ranging from sub-100nm to 10’s of microns.
The unique composition of these particles makes them ideal for sensitive electronics applications where they provide cost-effective polishing abrasive for slurry formulations, particularly CMP slurries. The product provides fast material removal rate and controlled surface finishes.
Other benefits of the product include the fact that it does not re-crystallize like colloidal silica and can be re-circulated to last longer. Applying the latest particle processing technology means that large particle counts can be controlled to substantially reduce defects on polished surfaces.