November 2001
Vol. 31, No. 11, pp 23–30.
Enabling Science

Table of Contents

Laurel M. Sheppard

Using "corrosion" to make ceramics

Form a metal precursor into a complex, shape, oxidize it, and get a ceramic with the same shape and dimensions.

Sometimes all it takes to solve a problem is to look at it from a completely different angle. For years, ceramists have been limited by their inability to form stable, complex shapes out of brittle metal oxides. Metallurgists, on the other hand, do everything they can to avoid forming metal oxides in the first place (they call them “corrosion products”). They can make any shapes they want out of malleable, ductile metals, but the resulting products don’t have the insulating and structural properties that ceramics have. When ceramic engineer Ken Sandhage joined a group of metallurgists, he didn’t see metal oxides as a problem, but as the beginning of a solution.

When Sandhage was earning his Ph.D. in ceramic engineering at the Massachusetts Institute of Technology in the early 1980s, he selected a metallurgist, Greg Yurek, as his adviser. Although he was the only ceramist in Yurek’s group at the time, this turned out to be an advantage—he gained more exposure than most ceramists to the theory of metal oxidation.

“Most work in this area has been done by metallurgists and electrochemists,” points out Sandhage, “so few ceramists know much about it, even though high-temperature metal oxidation is basically a ceramic process.”

Because metals are consumed by oxidation, metallurgists consider oxidation to be a type of corrosion that they would, of course, like to prevent. On the other hand, Sandhage realized that metal alloys that oxidize rapidly would be great as ceramic precursors or as nonfugitive binders. “This is using oxidation of metals from an opposite perspective. Instead of treating oxidation as metallic corrosion, oxidation can be considered to be an attractive processing route to ceramics or metal–ceramic composites,” Sandhage says. This idea became the basis for the development of the volume-identical metal oxidation (VIMOX) process and similar processes.

Laying the groundwork
But this would come later. First Sandhage had to earn his Ph.D., which involved studying the high-temperature corrosion of alumina in multi-silicate melts (slags) containing magnesia. Sandhage wanted to understand the corrosion of ceramic refractories used in coal gasifiers. Such refractories often contain Al2O3 that can react with the MgO in the slag to form spinel, MgAl2O4. He initially attempted to apply the Turkdogan–Wagner model for the active oxidation of volatile metals (1, 2) to describe the rate of this ceramic corrosion reaction, but he eventually realized a different model was needed.

He observed that in this ceramic reaction, Al2O3 dissolved incongruently in the corrosive slags; the Al2O3 dissolved by first forming a continuous, adherant layer of spinel that then, in turn, dissolved. Under steady-state conditions, the spinel formed and dissolved at equal rates, so that the spinel layer achieved a constant thickness on the Al2O3. Sandhage was able to describe this incongruent dissolution process with the use of the Tedmon model for the oxidation of chromium, which involves the simultaneous formation and volatilization of a continuous oxide layer with a constant thickness (3). As a result of his research, Sandhage realized that his knowledge of metal oxidation could be used effectively in modeling and understanding ceramic reactions and in developing new ceramic processes.

After working in the field of fiber optics for a few years, Sandhage joined his adviser at a start-up company called American Superconductor. Here, Yurek and his colleagues made oxide composite superconductors by selectively oxidizing yttrium, barium, and copper from alloys containing these elements and silver, resulting in a YBa2Cu3O7–Ag composite.

After 3½ years at American Superconductor, Sandhage decided to pursue oxidation-based research and develop other types of ceramics and ceramic composites in an academic setting. In 1991, he became a professor in materials science and engineering at Ohio State University, where he had to come up with a novel area of research. With his background in oxidation and a seed grant from the university, Sandhage started research in the oxidation of alloys containing alkaline and alkaline earth metals.

Sandhage went back to work done in the 1920s, when Pilling and Bedworth were looking at why oxidation rates differ in metals (4). Pilling and Bedworth considered the volume of oxide produced divided by the volume of metal consumed (now known as the Pilling–Bedworth ratio or PBR) to be an important parameter, and they evaluated this ratio for several metals. Most metals have a PBR > 1, whereas alkali and alkaline earth metals tend to have a PBR < 1 (Table 1).

Sandhage believed that if he could combine the right types of metals to form a precursor, so that the net volume change upon oxidation of the precursor was small or zero, he could convert complex-shaped metal-bearing precursors by oxidation into near net-shaped ceramic materials. (A net-shaped material has the same shape and size before and after processing. Most ceramics contain pores that cause them to shrink or deform when they are fired.) Metals tend to be ductile and relatively easy to form into complex shapes. On the other hand, ceramics traditionally have been difficult to make into complex parts without using expensive diamond machining to achieve the desired dimensions. By starting with easily formed metal precursors, then oxidizing the precursors to yield ceramics that retain the precursor shape and dimensions, near net-shaped ceramics can be produced without the need for ceramic machining. The VIMOX process is based on oxidizing different metals in a precursor (some of which expand, some of which contract upon oxidation) to limit the volume change upon conversion into a ceramic compound.

The VIMOX process
Sandhage and his students toiled for several years to demonstrate that the VIMOX process could be used to produce a number of technically useful ceramics and composites in near net shapes, such as those shown in Table 2 (6). During this time, Sandhage received support from several federal and state agencies for such research. In 1995, he was granted a patent for the VIMOX process (7).

The VIMOX process comprises several steps (Figure 1). First, precursors are prepared by mechanically alloying powders of metals and oxides to obtain a uniformly dispersed mixture with the proper phase content (for minimal volume change upon oxidation) and overall composition (for the desired final ceramic or ceramic composite). Next, the mixed powders are compacted and shaped by one or more of several metallurgical processes (e.g., pressing, rolling, drawing, machining). The amount of ductile metal in the precursor determines the type of fabrication process that is used. Approximately 30 vol% of ductile metal is used for pressing and about twice that for rolling and drawing.

Once the preforms are made into desired shapes, they are oxidized and converted into the final ceramic or ceramic composite using several heat treatments. These heat treatments can effect oxidation, compound formation, microstructural tailoring, or other desired processes. By carefully controlling the phase content of the precursor and the heat treatments, near net-shaped ceramics and composites can be produced without costly post-oxidation machining steps. Furthermore, because modest temperatures (300–500 °C) can be used to oxidize alkaline earth metals, the grain size of the resulting oxide can be quite small, so that subsequent sintering also can be done at modest temperatures. (Dense BaTiO3 has been produced at 1080 °C, several hundred degrees lower than normal, without a sintering aid.)

A casting process can also be used to prepare the precursors (Figure 2). A block of solid alkaline earth metal is placed in contact with a porous preform in the desired shape. The metal is melted under an inert atmosphere, causing it to infiltrate the pores of the preform. Heating the preform under oxygen produces a ceramic in the shape of the original preform that contains the alkaline earth metal.

One advantage of precursors containing alkaline earth metals is that they are ductile at room temperature, which makes them easy to machine and form into shapes. They are also easy to cast into shapes because of their low melting temperatures. Thus, the VIMOX process using these materials can make ceramics that retain shape, volume, and dimensions after oxidation, usually to within 1%.

In contrast, conventional ceramic processes require the use of organic binders that must be burned off the ceramic preform, producing high internal porosity. Such porosity produces significant distortions and shrinkage upon sintering, 20% or more, depending on the amount of binder used to form the ceramic preform (more for rolling, less for pressing). Since 1991, Sandhage’s group has produced a variety of functional ceramics by the VIMOX process, including biocompatible phosphates, ionically conducting cerates, dielectric titanates, superconducting cuprates, magnetic ferrites, and refractory aluminates and aluminosilicates for biomedical, sensor, electronic, magnetic, optical, chemical, and high-temperature applications.

Displacive compensation of porosity
One limitation of the VIMOX process is the thickness of parts produced. Parts thicker than several centimeters can take relatively long times (tens of hours) to oxidize completely. This limitation has recently been overcome with another oxidation-based process developed by Sandhage’s group, known as the DCP (displacive compensation of porosity) process. In 1998, graduate student Pragati Kumar was preparing preforms to spinel by infiltrating molten magnesium into porous Al2O3 and then solidifying the magnesium. He noticed that the Al2O3 had oxidized some of the magnesium to MgO during infiltration. After realizing that an oxidation–reduction reaction was occurring during infiltration, Sandhage and Kumar calculated the change in ceramic volume for this reaction:

3{Mg} + Al2O3 → 3MgO + 2{Al}

where {Mg} and {Al} refer to magnesium and aluminum dissolved in molten metal. To their surprise, more ceramic volume was generated than was consumed. They then decided to let this reaction run to completion during infiltration (reactive infiltration). They found that porous alumina preforms could be converted completely into dense, high MgO-bearing components with little change in shape or dimensions.

In other words, the increase in volume caused by the displacement reaction could be used to compensate for the porosity in the starting preform; hence, the term DCP. Because the oxygen source for such a displacement reaction is the solid oxide that is distributed throughout the preform, long-range diffusion of oxygen from outside of the preform is not required to complete this reaction. Once the molten metal fully infiltrates the preform (which occurs rapidly), the time required for complete reaction does not depend on the size of the preform. Sandhage’s group has since identified a variety of other volume-increasing displacement reactions that can be used to make components with a high ceramic content (Table 3).

The DCP process consists of two steps: infiltration of a molten metal into a porous, shaped ceramic preform and an in situ reaction of the molten metal with the ceramic preform (Figure 3). Because the volume of solid ceramic produced is larger than the original volume in the preform, dense composites with relatively high ceramic contents can be synthesized. If the metallic product of the reaction is solid at the reaction temperature, dense ceramic–metal composites are produced. If the metallic product is molten, the metallic liquid is squeezed out of the preform as the pores become filled with ceramic, and the composite contains a very high ceramic content. This latter process is called “pressureless reversible infiltration of molten alloys by the displacive compensation of porosity” (PRIMA-DCP).

A range of compositions and phase contents can be produced by varying the melt composition, the preform porosity, and the preform phase content. (The metal reinforcements in the composites can be continuous or discontinuous.) For example, composites of MgO/Mg–Al with MgO contents ranging from 70 to 86 vol% can be produced by varying the preform porosity from 47 to 29% (17). Composites with >80 vol% MgO were found to be electrically insulating; sufficient molten metal was extruded from the preform during reaction that only discontinuous particles of metal remained in the composite upon cooling to room temperature.

The DCP process also has been used to produce co-continuous MgAl2O4/Fe–Ni–Al composites by pressureless reactive casting of Mg–Al liquids at 900 °C into porous Fe/NiAl2O4 preforms with the net reaction:

4/3{Mg0.75Al0.25} + xFe(s) + NiAl2O4 → MgAl2O4 + y[Fex/yNi1/yAl1/(3y)]

where y = x + 4/3; 4 ≤ x ≤ 8; and the brackets {} and [] refer to liquid and solid alloys, respectively.

Composites with a variety of oxide and metal alloy compositions can be produced directly by DCP or PRIMA-DCP processes for use in electrical, refractory, insulating, or engine applications.

Nonoxide ceramic composites
The DCP process can also be used to make composites containing nonoxide ceramics. Sandhage recently worked with several senior undergraduate students to fabricate ZrC–W composites by the reactive casting of a Zr2Cu liquid into porous WC preforms (18). The net displacement reaction in this case is

0.5{Zr2Cu} + WC → ZrC + W + 0.5{Cu}

The residual molten copper does not form stable compounds with ZrC or tungsten and has minimal effect on the high-temperature resistance of the final material. Copper also has the advantage of lowering the reactive infiltration temperature and can be extruded out of the preform as the pores are filled. Dense, near net-shaped composites were produced within 2 h at 1200 °C and 1 h at 1300 °C.

Such composites are attractive for high-temperature applications where excellent resistance to creep, erosion, or thermal cycling is required, for example, throat inserts of rocket exhaust nozzles. Current nozzle liners are based on carbon or tungsten. Although tungsten exhibits minimal oxidation in solid-fuel rocket nozzles, and it has a very high melting point and good toughness, it is heavy (19.3 g/cm3) and relatively difficult to form into specific shapes at room temperature. Combining tungsten with ZrC is an alternative because ZrC is relatively lightweight (6.63 g/cm3) and chemically compatible with tungsten. This combination has better strength and toughness than ZrC alone.

However, the ZrC–W composites prepared by conventional methods are difficult to sinter, requiring hot pressing at 2000 °C and 20 MPa. They are also expensive to machine. Sandhage and his students have shown that the DCP process can produce ZrC–W composites in the desired shapes without machining at 1200 °C and ambient pressure. Furthermore, functionally graded ZrC–W composites can be produced by carefully tailoring the distribution of tungsten and WC in the starting preforms (19).

Sandhage has purchased a larger furnace for scaling up the DCP process. This furnace will be capable of making rocket nozzles that can be tested in rocket burner rigs at Edwards Air Force Base, CA (in collaboration with Wesley Hoffman of Edwards AFB).

Sandhage has also submitted a proposal to the U.S. Army to evaluate the use of the DCP process for making custom-tailored body armor. In this case, a plaster of Paris mold the exact shape of the torso is produced first. A slurry of silicon carbide is then cast into the mold. After drying, this porous preform is then infiltrated and reacted with a boron-bearing liquid to convert the porous SiC preform into lightweight, hard B4C–SiC composite armor that retains the preform shape and dimensions:

4{B} + (1+x)SiC → B4C + xSiC + {Si}

where {B} and {Si} refer to boron and silicon dissolved in a liquid solution. By tailoring the starting porosity and SiC particle size in the cast preform, Sandhage expects that near net-shaped B4C–SiC composites with tailored phase contents can be produced.

Although DCP is somewhat of an offshoot of VIMOX and can also produce net shapes at lower temperatures than conventional processes, there are major differences between the two approaches (Table 4).

Joint efforts expand applications
Sandhage is working with other Ohio State researchers to develop new applications for the VIMOX process. He has teamed up with Alan Litsky, a professor of orthopedics, to develop graded oxide coatings for metal hip implants. The pure oxide coatings of hydroxyapatite currently used do not adhere well to the underlying metal stem for long periods. A new graded hydroxyapatite coating concept has been demonstrated with the VIMOX process; the next step will be to make implants for animal studies after additional funding is obtained.

Sheik Akbar, a colleague in the materials science and engineering department, and Sandhage are investigating making TiO2-doped MgCr2O4 humidity sensors that can be reused after exposure to high temperatures. The VIMOX process produces near net-shaped sensors that can be positioned close to the ceramic ware to allow for local measurement of the drying rate. Placed on kiln cars, these humidity sensors could determine the drying rates of ceramic ware to allow for energy- and time-efficient rate-controlled drying.

One novel application of DCP takes advantage of marine biology, namely a type of green algae called diatoms. These single-celled creatures form microshells with intricate shapes and with submicrometer features. While on sabbatical in 1991 as a Humboldt Fellow in Germany, Sandhage met Australian marine biologist Monica Schoenwaelder, an expert on diatoms. After discussing her work, Sandhage came up with the idea of using diatoms as templates to make near net-shaped microdevices.

One application would be drug delivery biocapsules. Capsule-shaped diatoms, made of amorphous silica, have been reacted with magnesium to form MgO capsules that retain the diatom capsule shape. Because MgO is more biocompatible than SiO2,drugs could be easily and safely administered in MgO capsules. Capsule-shaped diatoms could also be converted into CaO, adding the benefit of dietary calcium to those who need it.

Eventually, Sandhage believes, genetic engineering can be used to tailor the diatom shape, and he has coined the term “genetically engineered microdevices” or GEMs. DCP would be used to tailor the composition and convert the silica into other ceramics or ceramic composites. Sandhage has applied for a patent (20) and expects to obtain seed money for further development.

“In addition to their size and shape advantages,” adds Sandhage, “diatoms have an extremely high replication rate of 3–8 times per day. This means you could theoretically make 1 billion similar microdevices in just 10 days. Genetically engineered diatoms could be used as biofactories to mass-produce large numbers of micro templates with similar tailored 3-D shapes that could then be converted by reaction into microdevices with tailored compositions for microsensors, micromotors, microrobots, etc.”

Bringing the products to market
After spending a decade conducting several millions of dollars worth of oxidation-based research and obtaining seven patents in metal oxidation technology, Sandhage is now actively working to commercialize the VIMOX and DCP processes. Orton Ceramic Foundation (Westerville, OH) is continuing development of the MgCr2O4 humidity sensors.

Sandhage believes that the VIMOX and DCP processes have great potential for commercially manufacturing ceramic and ceramic composite components with complex shapes. He warns, however, that “it is not easy to insert new processes into traditional ceramic manufacturing companies, especially since these companies are often not familiar with powder metallurgy or casting metallurgy-based processes. This requires a significant change in mindset.” Sandhage is thinking about starting his own company that will act as an intermediary to manufacture and test components.

Acknowledgments
Ken Sandhage has received research funding from the National Science Foundation, the U.S. Department of Energy, the U.S. Air Force Office of Scientific Research, and the Edison Materials Technology Center (Dayton, OH).

Back to the basics
The basic kinetic mechanism or mechanisms by which oxides undergo incongruent reduction with molten metals (DCP reactions) are not well understood. Sandhage and colleague Robert Snyder at Ohio State recently were awarded a National Science Foundation grant to study these reactions. The main objective of this research is to develop a fundamental understanding of the rate-limiting steps and microstructural evolution by which a solid oxide undergoes incongruent reduction with a reactive metallic liquid. A better knowledge of such reaction mechanisms will help predict how changes in processing conditions will affect reaction times, composite microstructure, and, consequently, the composite properties.

In situ X-ray diffraction (XRD) will be used to track, in real time, the formation of spinel (MgAl2O4) on Al2O3 surfaces immersed under molten Mg–Al layers. This is possible because the absorption of MoKα X-rays by Mg–Al liquids is relatively low. This is the first time that dynamic X-ray analysis will be used to study liquid metal–solid ceramic displacement reactions underneath molten metal (Figure 4).

The in situ XRD method requires making novel graphite heating cells that are transparent to X-rays. With these heating cells, the entire preform and surrounding melt can be equilibrated thermally, avoiding the steep thermal gradients normally seen when heating strips are used. Dynamic in situ thermogravimetric analyses will also be used to determine the steady-state rate of incongruent reduction under well- controlled conditions.

References

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  2. Wagner, C. Corros. Sci. 1965, 5, 751–764.
  3. Tedmon, C. S., Jr. J. Electrochem. Soc. 1966, 113, 766–768.
  4. Pilling, N. B.; Bedworth, R. E. J. Inst. Metals 1923, 29, 530–591.
  5. Encyclopedia of Materials: Science and Technology; Buschow, K.H.J., Ed.; Elsevier: New York, 2001.
  6. Sandhage, K. H.; Allameh, S. M.; Kumar, P.; Schmutzler, H. J.; Viers, D.; Zhang, X.-D. Mater. Manuf. Processes 2000, 15, 1–28.
  7. Sandhage, K. H. U.S. Patent 5,447,291, 1995.
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  12. Saw, E.; Sandhage, K. H.; Gallagher, P. K.; Litsky, A. S. Mater. Manuf. Processes 2000, 15, 29–45.
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  15. Jain, A.; Sandhage, K. H. In Innovative Processing and Synthesis of Ceramics, Glasses, and Composites IV; Bandal, N. P., Singh, J. P., Eds.; American Ceramic Society: Westerville, OH, 2000; pp 15–23.
  16. The Powder Diffraction File [CD-ROM], International Centre for Diffraction Data: Newtown Square, PA (2001 update).
  17. Kumar, P.; Sandhage, K. H. J. Mater. Sci. 1999, 34, 5757–5769.
  18. Dickerson, M. B.; Unocic, R. R.; Guerra, K. T.; Timberlake, M. J.; Sandhage, K. H. In Innovative Processing and Synthesis of Ceramics, Glasses, and Composites IV; Bandal, N. P., Singh, J. P., Eds.; American Ceramic Society: Westerville, OH, 2000; pp 25–31.
  19. Dickerson, M. B.; Snyder, R. L.; Sandhage, K. H., Presented at the 25th Annual Cocoa Beach Conference and Exposition, Cocoa Beach, FL, Jan 2001. Submitted for publication.
  20. Sandhage, K. H. U.S. Patent application pending.


Laurel M. Sheppard is a freelance writer based in Hilliard, OH (lashpubs@infinet.com). She has a B.S. in ceramic engineering from Ohio State University, Columbus.

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