Ceramic materials can take the heat, but repeated stresses will do them in because they are inherently brittle. When subjected to one too many stresses, ceramics will crack or even shatter, like Humpty Dumpty falling off the wall. The problem lies in tiny flaws that undermine the strength of ceramics. Voids or particles of the wrong size or shape that don't quite fit together can be the Achilles' heel of a ceramic, setting it up for eventual failure. The solution lies in the close packing of the particles that make up the material. Controlling the sizes and shapes of the particles that become the building blocks of ceramics is an essential first step toward developing highly reliable ceramics for energy applications.

Three ORNL engineers have developed a device that may help industry reinvent ceramic production. Called the electric dispersion reactor (EDR), the device produces ultrafine precursor ceramic particles of desired shapes and distribution of sizes. Such control could eliminate the tiny flaws that eventually grow into cracks in normally brittle ceramics, especially those containing multiple components. In addition, such control could eliminate the problem of misaligned grains, which limits the amount of electrical current that bulk superconducting ceramics can carry. Thus, this approach could improve the electrical current-carrying capacity of high-temperature superconducting materials.

In June 1992, a patent was issued to Michael Harris, Timothy C. Scott, and Charles H. Byers, all of ORNL's Chemical Technology Division, for the EDR. For this invention the three researchers were honored with an Inventors Award during the May 1993 Awards Night of Martin Marietta Energy Systems, Inc.

The EDR offers several advantages over the two conventionally used techniques--oil-water emulsion and spray pyrolysis--for making fine ceramic powders in the micron to submicron regime (a micron is a millionth of a meter). The EDR works well in converting aqueous metal salt solutions and metal alkoxides to hydrous metal oxides, oxygen-containing compounds dissolved in water. It has been used to produce silica, alumina, zirconia, titania, and the precursor powder for yttrium-barium-copper oxide, a high-temperature superconducting ceramic.

The EDR combines the features of homogeneous precipitation and the emulsion phase contactor, an invention of Scott, Robert M. Wham, and Byers that uses electric fields to improve the efficiency of solvent extraction--a separation method widely used by industry for recovering chemicals.

In solvent extraction, a substance is transferred from one liquid phase by dissolving it into a second one that does not mix with the first liquid. The movement of a substance from one liquid to another is called mass transfer. To increase the rate of mass transfer, conventional solvent-extraction systems agitate the mixture using mechanical mixers to increase the interfacial area by the formation of liquid drops. Considerable energy is required to agitate and mix both phases.

By subjecting the liquid mixture to high-intensity electric fields in the emulsion phase contactor, Scott and Wham obtained mass transfer rates 100 times greater than those achieved by mechanical systems. The increase in mass transfer was a result of a decrease in the size of the liquid drops, making possible an increased interfacial area. The ORNL system is also energy efficient, using 1% of the energy required by a mechanically agitated system. Because of its higher efficiency, the recovery method can be carried out in vessels one-tenth the size of those used in conventional solvent extractors. In 1990 the ORNL technology was licensed to Analytical Bio-Chemistry Laboratories of Columbia, Missouri, and National Tank Company of Tulsa, Oklahoma.

While Scott and Wham were developing the emulsion phase contactor, Harris was conducting research on homogeneous precipitation and particle growth. Homogeneous precipitation is the separation of a solid substance from a solution through a chemical or physical change. Under controlled conditions, the product is insoluble crystalline or amorphous particles that have a uniform size, shape, structure, and chemical composition.

Harris was studying particle production because his predecessor, Dave Williams, under the direction of Byers, was interested in measuring the viscosity of organic solvents at high temperatures. To accomplish this, he laced the liquids with 0.1-micron-size ceramic "seed" particles because they are rigid enough to withstand high temperatures. Because the ways that laser light shone on particles in a solution may scatter can be related to the Brownian motion of the particles, the technique could be used to measure liquid viscosity if the particle size is known. The technique can also measure particle size if the liquid viscosity is known. Hence, Williams and Byers later wanted to use dynamic light scattering to investigate the earliest possible stages of particle formation and growth. After the departure of Williams, Byers asked Harris to develop homogeneous precipitation techniques to generate silicon oxide (silica) and other metal oxide particles as small as possible and to study how they grow. Because well-defined powders are crucial to the development of advanced ceramics, this was a promising area of research.

Chemistry in a Drop

The homogeneous precipitation technique was used to easily form well-defined ultrafine powders of metal oxides containing one or two components. However, multicomponent metal oxide particles are difficult to synthesize by this technique. Thus, it became necessary to use physicochemical techniques to form ultrafine multicomponent metal oxide particles. In such techniques, particle size is determined by localization of the precipitation reactions in "microreactors" such as micron-size droplets.

While developing the electrically driven emulsion phase contactor, Scott and co-workers determined that micron-size droplets were formed during dispersion of aqueous drops (e.g., water) into an insulating organic phase (e.g., ammonia) by intense-pulsed direct-current electric fields. After reading some publications coauthored by Scott and after discussing matters with Byers and Scott, Harris suggested that the micron-size droplets serve as microreactors for the localized precipitation of metal salts and metal alkoxides. Thus, the electric dispersion reactor was conceived.

"In the EDR electrical stresses accumulate at the interface between the aqueous and organic solutions and atomize the aqueous phase into droplets at the spray nozzle," Harris says. "The result is a very fine mist. We can make particles of different sizes and shapes by changing the location and concentrations of the reactants on the droplets and by changing the intensity of the electric field."

Using the EDR, Harris and his colleagues have made particles of several different shapes (morphologies) as well as sizes. One particle type, dubbed the "porous prune," is made by precipitating silica during hydrolysis of tetraethylorthosilicate on the surface of the water-ammonia droplets and then drying them. Another particle, called a "PacMan particle" because of the hole on its surface, is formed by precipitation of hydrous aluminum oxide in droplets during transport of ammonia from the continuous to the dispersed phase.

EDR and Industry

The EDR is valuable for producing high-quality precursor ceramic powders for research purposes and possibly industrial production of ceramics. Very fine ceramic powders of known size, shape, structure, and chemical composition could be the basis for fracture-resistant ceramics. "The smaller the particles, the more you can control the microstructure and the less likely that the material will crack," Harris says. "In the EDR, we should be able to produce ceramic particles of a desired size or size distribution, whatever is needed to achieve an optimum packing density."

In January 1993 Harris and his associates achieved a milestone. By making 100 grams of zirconia powder in the EDR's largest production run, they showed that the EDR can produce a large enough quantity of material to suggest that it could be scaled up for industrial production. This production run was made possible by funding from the Department of Energy's Office of Basic Energy Sciences, Advanced Energy Projects, which has supported development of the EDR.

As a result of this success, Harris is trying to attract industrial interest from potential customers such as Du Pont, Dow Corning, and PPG. The group may also have a chance to work with Rohm and Haas and Dow Corning as part of the cooperative research and development agreements (CRADAs) involving use of the emulsion phase contactor and the EDR.

Harris is also collaborating with researchers in ORNL's Chemical Technology and Metals and Ceramics divisions. The precursor superconductor powder that Harris produces is of interest to Terry Lindemer of the Chemical Technology Division because of the ability to produce gram quantities of homogeneous powders in the EDR. Harris makes the precursor powder by dissolving yttrium nitrate, barium nitrate, and cuprate chloride in a water solution. This aqueous phase, which is sprayed into the EDR through a nozzle, is dispersed by the electric field to form tiny droplets. The ammonium in the continuous phase present in the reactor diffuses into the aqueous droplets, where it causes the three metal oxides to precipitate as fine powders containing yttrium (Y), barium (Ba), and copper (Cu). These powders will be sintered by Lindemer to produce yttrium-barium-copper (Y-Ba-Cu, or 1-2-3) superconductivity material. The main challenge for the EDR researchers is the formation of particles smaller than 0.1 micron.

Harris thinks that the ceramic industry should be attracted by the EDR's advantages over two competitive powder-producing techniques. One is spray pyrolysis, a generic term to describe a wide range of processes that generate ceramic particles directly from atomized precursor solutions or sols in a gas phase at elevated temperatures. The other is the water-oil emulsion technique in which mechanical agitation is used to form micron-size drops that serve as microreactors. In this case, Harris says, considerable energy is used to "beat" the fluid into micron-size particles, which are made even smaller by use of surfactants (detergentlike substances that reduce surface tension) that later must be removed.

EDR's Advantages

According to Harris, the EDR offers these advantages over the conventional techniques:

  • It will potentially use about one-tenth as much energy as the water-oil emulsion technique because no stirring mechanism is required. Only a small amount of electricity is needed to spray the aqueous phase from the nozzle and create a field to disperse it. (ORNL researchers will do an economic analysis in 1994 to calculate the precise energy savings of using the EDR instead of conventional techniques.)

  • Surfactants are not needed.

  • No moving parts are used, so much less maintenance is required.

  • Because soft agglomerates are formed in the EDR, it offers a potentially higher yield than spray pyrolysis.

  • Ceramic processing in the EDR is dust-free because it is contained in a slurry phase; thus, it will present reduced adverse health impacts because no breathable particles will be released into the air.

  • The solvent will be recycled through a continuous flow scheme, minimizing discharge of waste products.

  • By connecting modular units in a series, the EDR can be scaled up more readily than spray pyrolysis.

Does the EDR have any disadvantages? "Some products might require a conductive continuous phase," says Harris. "In the EDR the continuous phase must be a nonconductive or insulating fluid. A conductive fluid would be heated up by current flow, thus wasting energy."

Improving the EDR

Harris says he and his colleagues are considering using small amounts of a surfactant to reduce particle size and the energy needed to disperse droplets. A surfactant would lower the surface tension at the interface between the aqueous metal oxide and inorganic liquids, thus decreasing the energy required to atomize the aqueous phase.

To further enhance EDR performance, Harris and his associates are also trying to improve the spray nozzle design. "Osman Basaran, a group leader in the Chemical Technology Division, and I have done theoretical calculations to enhance nozzle design, and our results have been submitted for publication in the Journal of Interfacial Science," Harris says.

He explains that the effects of an electric field on droplets was first studied systematically by Zeleny in 1915. He was interested in understanding mechanisms of charge separation in thunderclouds and its effects on the shapes and instabilities of soap bubbles.

Harris says that it has long been known that droplets accumulate charges on their surfaces, causing them to deform in an applied electric field. "We study the point at which a static drop becomes unstable in an electric field. We found that the energy required to cause a droplet to break apart into a spray decreases as nozzle length increases up to a point. Utilization of a particular nozzle-electrode geometry could result in a more efficient use of electrical energy."

As industry searches for a fracture-resistant ceramic for energy applications, ORNL's electric dispersion reactor may be the key to reinventing the ceramic manufacturing process to make more durable and efficient products.

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