ONLINE EXCLUSIVE: Beneficial Beryllia

January 11, 2006
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Beryllia (BeO) is valuable for producing ceramics with a high thermal conductivity, particularly in the lower temperature ranges (below 300ºC). BeO ceramics are finding applications in a range of advanced technologies.

Green machined isopressed BeO parts.


Beryllia (BeO) is valuable for producing ceramics with a high thermal conductivity, particularly in the lower temperature ranges (below 300ºC). Its thermal conductivity is about 15 times that of dense alumina at ambient temperatures but can be higher depending on purity-the purer the ceramic, the higher the thermal conductivity. BeO has excellent dielectric properties and outstanding resistance to wetting and corrosion by many metals and non-metals. Its mechanical properties are only slightly less than that of 95% alumina ceramics. Like alumina, BeO is readily metallized by thick- and thin-film techniques.

Although much has been written about the real and perceived health and safety concerns of beryllia, these concerns are often misunderstood. Manufacturers of BeO ceramics must take special precautions and follow all material safety guidelines, but handling beryllia ceramic in its solid form poses no special health risks. Additionally, beryllia ceramic parts are often fully recyclable.

Table 1. (Click image for larger view)

During the last four decades, BeO ceramics have provided the electronics designer with attractive properties such as high thermal dissipation, a low coefficient of thermal expansion, high electrical insulation and a low dielectric constant (see Table 1).1 High-performance BeO parts have been manufactured using almost all common ceramic technologies, including dry pressing, 2 hot pressing, isostatic pressing, extrusion, tape casting3 and slip casting. However, this article focuses exclusively on recent manufacturing advances achieved through the use of the isopressing forming technology at Brush Ceramic Products (BCP) in Tucson, Ariz. 4,5 Typical BeO parts manufactured using this technology include blocks, disks, tubes, rods, crucibles and ion laser tubes.

Figure 1. A Sweco M60 vibratory mill.

Spray Drying

To achieve consistent high-quality BeO ceramics, high-purity beryllia powder is dispersed to an average size of 0.6-0.7 microns by milling in water and in the presence of a dispersant prior to spray drying. Sufficient powder deagglomeration is mandatory to eliminate unwanted porosity in the fired ceramic. Deagglomeration at BCP is accomplished in a Sweco M60 vibratory mill, as shown in Figure 1.

Figure 2. A Bowen No. 1 tower dryer.

After adding a binder to hold the agglomerate's integrity, the slurry is spray dried in a Bowen No. 1 tower dryer (see Figure 2) to produce a homogeneous, well-dispersed, flowable powder. Less than 1% by weight total magnesia and silica are added during milling as a sintering aid. The main purpose of spray drying is to generate a flowable powder for automatic dry pressing and to provide a pressable powder with a homogeneous dispersion of sintering aids. Spray drying has a direct impact on powder flowability, compaction behavior (tap density), sinterability, shrinkage, green strength and fired density. Polymethacrylic acid is used as a dispersant, with a polyvinyl alcohol or a non-ionic self-crosslinking acrylic emulsion used as binders. 6 The spray dried agglomerates with an average size of 60 microns are homogenized and conditioned with a lubricant and an antistatic prior to pressing.

Isopressing

Cold isostatic pressing (CIP) has long been established as a proven method of consolidating powders into a given shape, particularly when the required shape has internal and/or external profiles. Brush Ceramic Products has used the CIP technique to form beryllia parts from spray-dried powders for many years. The technique consists of loading a mold, usually made out of polyurethane, with spray-dried powder, sealing it, and submitting it to very high pressure in the isopress vessel for a given period of time.4 After decompression, the green part is removed from mold and fired in a conventional beryllia air firing cycle at temperatures that can reach 1550ºC.

One objective of isopressing is to achieve a green density that is uniform throughout the green body and high enough to achieve a workable green strength, as well as the required sintered density. Large and/or complex shapes can be produced using CIP, and both internal and external features can be incorporated into the green body. Isopressing coupled with green machining yields near-net-shape products. Using CNC equipment, a repetitive high process capability (Cpk = 6.4 in an igniter insulator machining operation) and very tight tolerances (7 microns repeatability) can be held. Typical part shapes produced on an industrial scale include blocks, discs, tubes, rods, crucibles, laser tubes and aircraft igniter insulators.

Figure 3. The National Forge Isomax 30 cold isostatic press with 4970 MPa (30 Kpsi) capability and a 12-in. diameter by 36-in.-long chamber used to produce the isopressed parts at BCP.

Equipment used to produce the isopressed parts at BCP is shown in Figure 3. It consists of a National Forge Isomax 30 cold isostatic press with 4970 MPa (30 Kpsi) capability and a 12-in. diameter by 36-in.-long chamber. Typical part shapes generated on an industrial scale include blocks and discs (7 in. maximum diagonal by 28 in.-long), tubes and crucibles (0.020 in. to 6 in. ID), rods (high aspect ratio), ion laser plasma tubes (multi-hole tubes), and igniter insulators (profiled ID and OD).

Figure 4. The effect of the isostatic pressure on fired density for the standard BeO body. (Click image for larger view)

The isopressing process involves loading the mold, usually made out of polyurethane or natural latex rubber, with powder and sealing it. De-airing is required on low green strength materials. The mold is transferred into the isopress vessel and the vessel is sealed. The liquid medium (water) pressure is raised, and the parts are allowed to dwell at a peak pressure that can reach 17.5-29.5 Kpsi. Figure 4 shows the effect of the isostatic pressure on fired density for the standard BeO body. After decompression, the mold is removed and the green compact is removed from the mold.

Figure 5. Green density vs. dwell time at 3000 MPa peak pressure and room temperature. Powder A contained 3% binder, while Powder B contained 6% binder. (Click image for larger view)

To minimize excessive powder usage and green machining, a consistent mold filling must be achieved from part to part. This is accomplished by using a computerized weigh feeding system that minimizes variability to less than +/- 0.1 gram. At the same time, proper loading of the mold by using vibration ensures constant packing density of the powder and dimensional consistency of the compact. 4

Figure 6. The effect of the powder temperature on green density. (Click image for larger view)

Figure 5 shows a plot of green density vs. dwell time at 3000 MPa peak pressure and room temperature. It indicates that the dwell time has a pronounced effect on green density initially and a relatively smaller effect after extended dwell times. Figure 6 shows that the temperature of the powder, at 3000 MPa and 20 sec dwell, also has an effect on green density. This has been discussed by DiMilia and Reed7 and is an effect of the glass transition temperature of the binder being used. For the system described in this article, the importance of controlling the temperature during forming and dwelling is easily seen. In practice, this is accomplished by keeping the loading station under temperature and humidity control and using an automatic dwell timer.

Dimensional tolerances are typically +/- 6% for the as-pressed parts. However, ñ2% can be achieved through the careful design and use of tooling, and controlling the compression ratio of the powder. The surface finish characteristics of the parts will be affected by the surface finish of the tooling used, the size distribution of the spray dried powder, the degree of deagglomeration of the slurry prior to spray drying, and the raw beryllia particle size distribution.

Green Machining

When the as-pressed tolerances are not acceptable to the final part requirements, green machining is used to allow tolerances to +/- 1% (0.5% typical). Several cost advantages are derived from using green machining, including faster stock removal compared to post-fired machining; near-net shapes as fired (+/-1%), which minimizes or eliminates post-fired machining; and the ability to produce near-net shapes even in small lot sizes. While near-net shapes can be achieved through isopressing alone, a high tooling cost can make this method too expensive to be practical.

Figure 7. Examples of the variety of features that can be green machined. (Click image for larger view)

Examples of the variety of features that can be green machined are shown in Figure 7. Just about any lathe or mill operation can be performed in the green state, as long as a few requirements are maintained. For example, the workpiece must be held in the machine tool without being damaged. Bending, crushing and cracking are all possible problems, but these can be avoided by ensuring that the workpiece has a high green strength and by using a minimum chucking pressure (1-2 MPa max), along with vacuum arbors to mount the workpiece, where possible, and rubber-lined jaws or fixtures.

It is also necessary to ensure that the beryllia waste material that is machined away is captured at the source. Ventilation systems with a 250- to 450-meter-per-minute capture velocity can be used to achieve a balance between optimizing the material removal rate and capturing the cuttings, thereby helping to minimize operating costs.

The machine tool should also be selected to minimize costs. This is true even when run sizes are moderate. The machine tools most commonly used for green machining of beryllia at BCP are standard or CNC lathes and standard or CNC end mills. While standard equipment is relatively inexpensive, it is more appropriate for small runs where simple and fast setups are all that is required. Using CNC equipment can provide advantages such as repetitive machining (high process capability index8 [Cpk = 6.4 in an igniter insulator machining operation]), tight tolerances (typically 7 microns repeatability is maintained) and higher production rates. For example, one milling operation went from 10 minutes in a standard mill to 30 seconds in a CNC mill. This is especially true when the job requires a multi-tool setup. CNC machining is also best when complex profiles are required on either the inside or outside diameter of the part.

It has also been found that using diamond-tipped cutting tools is advantageous to resist the abrasiveness of the ceramic. Even tungsten carbide tools do not remain sharp for more than a few parts.

Firing

After the parts have been green machined, they are ready to fire. During firing, several factors must be considered, including the shrinkage variation (controlled by the compact green density, which is driven by the pressure during forming); distortion and warpage (enough machining stock must be left so that the part can be cleaned up during post-fired machining); fired ceramic grain size (typically controlled by the kiln temperature profile and by the placement of the parts in the kiln); and binder burnoff (parts may crack if the debinding rate is excessive).

At BCP, the primary method of controlling the firing is through using control discs-dry, pressed discs of a known green density (within +/- 0.1 %) and shrinkage for a given kiln cycle-that are placed throughout the kiln. Statistical process control techniques are used to monitor the shrinkage and grain size of the discs2 to determine if the kiln was functioning properly during the cycle or if any variations occurred.

The secondary means of controlling the firing is to use curves of the forming pressure vs. shrinkage for a given powder lot. This allows the forming pressure to be set to match the design shrinkage of the tooling used. However, this effort can be minimized when the variation in shrinkage between powder lots is minimal. At BCP, one pressure is assigned to a given tool and rarely requires changing. Several BeO parts using this manufacturing technology are shown in the photo at the beginning of this article.

Expanding Applications

BeO presents special properties such as chemical inertness for crucibles for Li-ion batteries, high electrical insulation for igniters, and high electrical insulation and high thermal dissipation for microelectronics and optoelectronics packaging, making it the candidate of choice for these applications. It also offers an exceptionally low thermal neutron absorption cross section, which makes it a good neutron moderator (for slowing down fast neutrons) in fission reactors, as well as a promising material for high-temperature nuclear reactors. BeO has been used as a neutron reflector to reduce the size of reactor cores, and it is used in nuclear weapons for the same purpose.

High-precision, machined, large BeO bodies are used as ion laser plasma tubes (bores or plasma envelopes for gas lasers); helix supports; collector insulators; traveling wave tubes; Klystron tubes; igniter insulators; reactor components and other structural components; electron tube parts; radiation and antenna windows; radar antennae; and crucibles for melting uranium, thorium and beryllium metal.

Chemically stable, strong and rigid BeO parts are fabricated using a combination of ceramic powder preparation, spray drying, isopressing, green machining and high-temperature sintering. Key parameters controlling the process are the green density and strength of the BeO material, careful workpiece handling, appropriate dust control and controlled firing. The resulting parts can then be metallized using standard thick-film metallization, plating and brazing to attain ceramic-to-metal bonding if needed in downstream manufacturing. c

For More Information

For more information about BeO ceramics, contact Brush Ceramic Products, 6100 S Tucson Blvd., Tucson, AZ 85706; (520) 746-0251; (520) 294-8906; e-mail lee_vandermark@brushwellman.com or sepulveda@iamincaz.com; or visit www.brushceramics.com.

References

1. Kaczynski, D.J. and Sepulveda, J.L., "Beryllia Production of Ceramic-Grade Powders," Mineral Processing and Extractive Metallurgy Review, Vol. 13, 1994, pp. 77-88.

2. Sepulveda, J.L., Jech, D.E. and Ferguson, G.P. "Using SPC for the Dry-Pressing of Beryllia Parts," Amer. Cer. Soc. Bull., Vol. 73, No.1, Jan. 1994.

3. Sepulveda, J.L. and Kottman, R.E., "High Performance Electronic Substrates From Tape Casting of Beryllium Oxide," Ceramic Transactions, Vol. 26, Forming Science and Technology for Ceramic, 93rd Annual Meeting and Exp. of the American Ceramic Society, Cincinnati, Ohio, April 1991, pp. 211-218.

4. Kovell, G.A. and Sepulveda, J.L., "Optimizing Green Machining after Isopressing of Beryllia Ceramic Bodies," Ceramic Transactions, Vol. 26, Forming Science and Technology for Ceramics, 93rd Annual Meeting and Exp. of the American Ceramic Society, Cincinnati, Ohio, April 1991, pp. 231-239.

5. Kovell, G.A.; Sepulveda, J.L.; and Sims, T.D., "Effect of Forming Parameters on the Fired Material Properties of Isopressed Beryllia Ceramic Bodies," 95th Annual Meeting and Exp. of the American Ceramic Society, Cincinnati, Ohio, April 18-22, 1993.

6. Jech, D.E. and Sepulveda, J.L., "Advances in Beryllia Sintering," Sintering 1995, Penn State University Park, Sept. 24-27, 1995.

7. DiMilia, R.A. and Reed, J.S., "Dependence of Compaction on the Glass Transition Temperature of the Binder Phase," Amer. Cer. Soc. Bull., Vol. 62, No. 4, 1983, pp. 484-488.

8. Kane, V.E., "Process Capability Indices," Journal of Quality Technology, Vol. 18, Jan. 1986, pp. 41-52.



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