Melt Cools Into Final Solid

al's microscopic structure may have a smooth transition, for instance, from a strong metal on one end to a heat-resistant ceramic on the other. With a gradual variation in composition, rather than abrupt boundaries between different compounds, graded materials can better resist dramatic changes in temperature and mechanical stress.

Taking a Closer—and Quicker—Look

A lthough combustion synthesis offers a number of potential advantages over conventional techniques, scientists must overcome some hurdles before it can enjoy widespread use. Customizing materials requires a detailed mechanistic understanding of how the new molecular structure forms—and that means being able to track what happens during each phase of the reaction. The most attractive features of combustion synthesis—high heating rates, high temperatures and short reaction times—also make it difficult to study the reaction wave as it propagates through the mixture. In response to this challenge, a new field of fundamental research is emerging.

Engineers are beginning to incorporate ideas from various areas of science and engineering into their investigations of combustion reactions. From biologists who study fast muscle movement, engineers borrowed a technology with which they can monitor precisely when certain components melt and new crystals form during a reaction. In this method, called time-resolved x-ray diffraction, a machine scans the reacting sample with powerful synchrotron radiation to generate x-ray patterns of the material every 0.01 second. From these pat terns, researchers can identify the composition of chemical phases that appear at a particular moment during the reaction. To identify the evolution of the microstructure, engineers employ a technique first developed to study the way solid rocket fuel burns. By dropping a burning sample into a pool of liquid argon, researchers can freeze the reaction before all the starting components have been transformed into the final product. They then slice this quenched sample into thin layers and analyze its microstructure using techniques such as electron microscopy.

As engineers began looking at combustion reactions with these new methods, they discovered that the characteristics of the material change as the reaction wave moves through different zones. Many researchers now concentrate their efforts on understanding phenomena near the leading edge of the reaction wave—also called the combustion front—be-cause the primary compounds that define the structure and properties of the final material form there. The width of this zone generally varies from 0.05 to 0.5 millimeter, enough to hold only a few to tens of particles. Within this tiny area, several physical and chemical processes work in concert to transform the molecular structure, so researchers needed a way to monitor this specific location.

During the past six years, my colleagues and I at the University of Notre Dame have designed a tool that is especially useful for observing microscopic conditions along the combustion front. We can now watch the reaction wave pass through a material with the help of a high-speed digital video camera that peers through a microscope and sees objects as

High-Speed Video Microscopy

Improving a Reaction

Temperature

Low Initial Density

Medium Initial Density

Low Initial Density

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^i^ot^pcts

n s

Titanium and ilicon powders

Titanium silicide

il

Medium Initial Density

High Initial Density

Ideal Reaction Conditions

High Initial Density

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Ideal Reaction Conditions

SCINTILLATING WAVE characterizes many combustion-synthesis reactions, including the formation of titanium silicide. Four high-speed video frames show that silicon melts and surrounds solid particles of titanium, initiating a reaction (red).The heat wave moves forward only as these "hot spots"appear (a).This highly variable temperature at the leading edge of the reaction, also called the combustion front, is a problem because it causes nonuniformity in the microstructure of the final product. Starting a reaction in samples of higher density,which can more efficiently dissipate heat, decreases the occurrence of hot spots (b and c) and results in a more uniform microstructure. Researchers strive for reaction conditions that yield a quasi-homogeneous wave,which moves steadily and shows relatively little variation of temperature along the combustion front (d).

small as 0.0015 millimeter in diameter (V50 the thickness of human hair). Our camera captures up to 12,000 frames per second; a conventional video camera captures only 30 frames per second. Using this technique, we have discovered that a reaction that appears to travel steadily on visible scales may move in a complex, unsteady fashion on the microscopic level.

Given this new information about the microstructure of the transforming material, we classified the reaction waves into two general types: quasi-homogeneous and scintillating. A quasi-homogeneous wave moves steadily, and the temperature varies relatively little along the combustion front. Such conditions are ideal for making a material with a highly uniform structure. The combustion front of a scintillating wave, in contrast, displays extreme temperature variation, which may lead to flaws in the final solid. This wave pattern occurs in systems in which at least one reac-tant melts during the reaction. Particles of the reactant with the lower melting point begin to melt just ahead of the combustion front, and the reaction wave moves forward only as these "hot spots" appear.

Characterizing waves in such detail has become possible only in the past two years, but what my colleagues and I find most exciting is that knowing these details allows us to precisely tailor properties of the final compound. In reactions that travel as scintillating waves, we can control the appearance of hot spots by carefully selecting the experimental conditions. Reactions between titanium and silicon, for example, produce fewer hot spots—and therefore less temperature variation along the combustion front—when we increase the density of the pellet of starting powders [see illustration on opposite page].

New ways of analyzing combustion reactions have opened the door for scientists to invent new materials more efficiently. One such example is the enhancement of cobalt-based alloys, widely used in orthopedic implants such as artificial hips and knees. For decades, the technology for making im-

Combustion Synthesis

What It's Good For

COMPOSITION

Carbon plus an element such as titanium or silicon (TiC,SiC)

Abrasives,cutting tools, ceramic reinforcements

BORIDES

Boron plus a metal such as titanium or lanthanum (TiB2, LaB6)

Abrasives,cutting tools,cathodes

SILICIDES

Silicon plus a metal such as titanium or molybdenum (TiSi2,MoSi2)

Heating elements, electrical connectors, Schottky barriers for electronics

ALUMINIDES AND TITANITES Aluminum or titanium plus a metal such as nickel (AlNi,TiNi)

Aerospace and turbine materials, shape-memory alloys

NITRIDES

Nitrogen plus an element such as niobium or silicon (NbN, S13N4)

Ceramic engine parts,ball bearings, nuclear safety shields

HYDRIDES

Hydrogen plus one or more metals (MgH2,ZrNiH3)

Hydrogen storage, catalytic materials

OXIDES

Oxygen plus one or more metals (YBa2Cu3Ö7-x, La0.8Sr0.2CrO3)

High-temperature superconductors, gas sensors,fuel cells

CHALCOGENIDES AND PHOSPHIDES Sulfur or phosphorus plus a metal such as molybdenum or gallium

High-temperature lubricants, semiconductors

plants has involved melting ingots of the alloys in furnaces and pouring them into molds of the proper shape. My colleagues and I are working both to make stronger alloys and to eliminate manufacturing steps by synthesizing the implant directly within its mold. Developing this one-step technology—and other applications of combustion synthesis—will take several years, but high-speed video microscopy and other methods for tracking these ultraquick reactions may bring combustion synthesis to the forefront of materials production technology. gg

The Author

Further Information

ARVIND VARMA has been investigating the combustion synthesis of advanced materials in his laboratory at the University of Notre Dame for the past 10 years. His interest in other areas of chemical engineering began at the University of Minnesota, where he received his Ph.D. in 1972. He worked for two years at Union Carbide before joining the Notre Dame faculty in 1975. There he has served as Arthur J. Schmitt Professor since 1988.

Self-Propagating High-Temperature Synthesis: Twenty Years of Search and Findings. A. G. Merzhanov in Combustion and Plasma Synthesis of High-Temperature Materials. Edited by Z. A. Munir and J. B. Holt. VCH Publishers, 1990.

Combustion Synthesis of Advanced Materials: Principles and Applications. A. Varma, A. S. Rogachev, A. S. Mukasyan and S. Hwang in Advances in Chemical Engineering, Vol. 24, pages 79-226; 1998.

Complex Behavior of Self-Propagating Reaction Waves in Heterogeneous Media. A. Varma, A. S. Rogachev, A. S. Mukasyan and S. Hwang in Proceedings of the National Academy of Sciences USA, Vol. 95, No. 19, pages 11053-11058; 1998.

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