A new class of glass-ceramics has been developed for micro-milling media.

The use of mechanical energy in the manufacturing of inorganic-based products is well established. Throughout major stretches of history, the technology has primarily been restricted to morphological modifications of naturally occurring minerals, such as the refinement of ores and rocks. Over the years, grinding technology has evolved toward direct size reduction, with the “cutting edge” being several micron particles in the 1960s, 1 micron in the 1980s, 100 nm in the 1990s, and currently progressing toward sub-10 nm.

This evolution coincided with an explosion of industrially important applications using the unique properties of nanomaterials. Thus, an interesting conjuncture has formed in recent years. The mechanical path to high-surface-area materials could be at least as feasible a synthesis path as other traditional methods of small particle preparation (e.g., solution-derived, gas-phase condensation, etc.). Indeed, direct milling has been rapidly expanding, driven in part by demand in the following areas:

• Pigment dispersions for color filters (latest generation LCDs)
• Ink jet products of the highest quality level
• Nano-additives with various functional properties
• Functional ceramics (e.g., for multilayer ceramic capacitors)
• Nanodispersions of metals
• Pigment preparation

Why Micro-Milling Media?

In high-energy bead mills, which are the most common manufacturing method for fine particle dispersions, stirring’s kinetic energy is absorbed by grinding media and then exerted upon a milled product. The extent of the milled product’s size reduction strongly depends on both the number of contacts between the grinding bead and milled product per unit of time, and the intensity of these contacts. Correspondingly, the generation of sub-100 nm particles benefits from using milling media in sizes of 100 microns and below.

One example taken from research presented by Bühler AG demonstrates that a three-fold reduction in the ultimate particle size can be attained by use of 90-micron media instead of 300-micron media (see Figure 1).¹ In addition, milling efficiency, which is defined as the energy input required to reduce the particle size of a certain fraction of a slurry to a desired final value, increases as the milling media size decreases. Figure 1 demonstrates that by using 90-micron media instead of 300-micron media, a 40 nm particle size can be generated at half of the required mass-specific energy.

As with most technologies, adoption depends on the existence of cost-effective production methods. For the production of nanoscale dispersions, the media cost often becomes a limitation, especially in the size range of 100 microns and below. For example, at a price for micro-milling media of $500-600/kg for 100 microns and up to $1000/kg for 50-micron beads, the cost of media required to fill a 10 L production mill can be comparable to the cost of the mill itself.

Currently, only one type of ceramic material, yttria-stabilized zirconia (YSZ), has the performance characteristics adequate to meet the stringent contamination demands of micro-milling applications. The wear resistance of milling media should be extremely good so that nanoparticulate dispersions do not become contaminated by media wear debris.

Zirconia is chosen for its high strength; its resistance to breakage is among the highest for all ceramic media materials currently produced. In addition, its specific gravity (among the highest at around 6 g/cc) affords high milling intensities and helps in resisting hydraulic packing. Hydraulic packing in a stirred mill occurs when drag forces on the media due to product flow overcome the natural turbulence of the media, causing beads to pack around the mill’s discharge screen. High media costs, along with the general difficulties of handling smaller beads resulting from spillage concerns and poor flowability, are often cited as barriers to the broader adoption of small bead milling technology. Yet markets show a healthy appetite for nanomaterials produced via milling.

Nanocrystalline Glass-Ceramic

Due to the demand for affordable nanomaterials, a new class of rare-earth-aluminate-zirconate glasses with high amounts of Al₂O₃ and ZrO₂ (U.S. patent US7563293) has been developed. Such glasses, when crystallized by subsequent heat treatment, yield beneficial mechanical characteristics such as strength and hardness.

With the emergence of a need for cost-effective, high-performing micro-milling media, a composition that demonstrates the right combination of properties required for micro-milling media was also developed. The composition for this product is (wt%) 45 La₂O₃, 20 Al₂O₃, 30 ZrO₂, 5 TiO₂.* (This product is currently being field tested. It is an experimental product that has not been introduced or commercialized for general sale.) Processing of these materials starts with the formation of glass microspheres, as shown in Figure 2.

To attain the highest mechanical characteristics and density, the glass is crystallized via a routine heat treatment process. Due to the highly metastable nature of a “kinetically frozen” glass state, conversion to nanoscale glass-ceramic occurs without the need for nucleating agents. The conversion takes place in three distinct crystallization events, as shown in the inset to Figure 3.

The highest density (~ 5.8 g/cc) is achieved after the third exothermic event is completed. Overall structural compaction upon conversion from the glass to the crystalline state is about 11.5 vol%, which translates into about four linear percent shrinkage. This densification has nothing to do with the traditional sintering compaction that takes place during the preparation of YSZ or other crystalline ceramic materials. In the latter case, porosity introduced during a prior manufacturing step is eliminated first through atmospheric sintering and then, optionally, through hot isostatic pressing. In glass crystallization, there is no initial porosity and the compaction is purely “structural” in nature. Interestingly, some volumetric expansion occurs in these materials when heat treatment temperatures greatly exceed the third exothermic peak temperature, as evident in Figure 3.

The microstructure of glass-ceramic material obtained via crystallization of the parent glass is distinctly nanocrystalline. As shown in Figure 4, the initial grain size of a few tens of nanometers coarsens to a maximum size of about 100 nm after heat treatment temperatures just exceed the third exothermic event. Near-complete elimination of the glassy phase is achieved at this point, as evidenced by microstructural and density analysis. In this case, the present glass-ceramics differ from conventional systems with common glass formers (e.g., SiO₂), in which a residual glassy phase amounting to 2-10 vol% always remains. As a comparison, Figure 4(C) demonstrates that the microstructure of YSZ used for micro-milling media is significantly coarser, with grain sizes typically ranging from 0.2-0.5 microns.

Fine microstructures and the corresponding reductions in grain size-related flaw dimension are typically cited as a prerequisite for high mechanical properties in ceramic materials. This holds true for most ceramics on the marketplace today, such as Al₂O₃ and ZrO₂ and, as was discovered, also applies for the new material. Figure 5 demonstrates that attainment of the highest strength and hardness characteristics coincides with the development of sub-150 nm size microstructure and the elimination of the residual glassy phase.

Further heat treatment to temperatures greatly exceeding the third exothermic crystallization peak results in a decrease of mechanical characteristics due to the onset of significant grain coarsening, consistent with commonly acceptable theories in ceramic materials. Interestingly, very high crush strength (> 1200 MPa) is obtained in as-prepared glass spheres. This phenomenon is attributed to surface compression induced via rapid cooling, similar to the “thermal tempering” effect in regular glass. As a comparison, data from commercial premium YSZ beads demonstrates that the present glass- ceramics exhibit comparable (if not higher) strength and higher hardness.

Performance Considerations

A typical method for evaluating the performance of milling media involves a wear test in which beads are run in a high-energy bead mill under fairly aggressive conditions. During the test, the mass loss of the media at predetermined time intervals is measured. The result of such a test conducted by an independent third party shows excellent performance of the new material (see Figure 6).

The higher initial wear rate of the glass-ceramic material is likely related to a “conditioning” effect in which finer and weaker beads are eliminated. This typically occurs within the first hour of running a media mill and is a commonly known effect in the industry.

By tailoring the composition and heat treatment profile, a novel nanocrystalline glass-ceramic bead material that demonstrates both high specific gravity and excellent mechanical characteristics has been developed. The composition and processing method is distinctly different from that of YSZ, but the mechanical properties and milling performance are distinctly similar. The employed manufacturing method is especially suited for micro-sized beads and is designed to be very cost effective. Based on these results, the overall performance of the new media in high-energy mills is expected to be comparable to the more widely known and more expensive YSZ media.

For additional information, contact Amy Barnes, Ph.D., Materials Specialist, 3M Energy and Advanced Materials Division, 3M Center, Bldg. 236-GB-27, St. Paul, MN 55144-1000; call (651) 736-3477; or email asbarnes@mmm.com.

Reference

1. Brochure for Bühler MicroMedia™ Mill. (MicroMedia is a registered trademark of Bühler AG.)

*Referred to commercially as 3M™ Micro Milling Media ZGC.

Links

• 3M