Johns Hopkins Whiting School of Engineering 1996 Annual Report [text version]

Whiting School of Engineering 1996 Annual Report

Materials Science and Engineering
Materials That Can Take the Heat
Giving ‘Smart’ Materials a Shock and a Shove
Phosphors for the Home and Office
Department Facts

Materials That Can Take the Heat
Assistant Professor Timothy Weihs works in a nano-world, where measurements are made in atoms instead of inches. Specifically, he studies, designs, and develops multilayered materials with scientific and commercial applications. Take computers, for example. Break one down to its smallest components and you have a vast array of parts, including chips. Now, look at the lines on the backside of any chip. This is one of the Lilliputian environments Weihs enjoys exploring.

“In this research project, we look at thin film metallizations for integrated circuit manufacturing,” Weihs begins. “Currently, the industry uses aluminum for the lines on a chip and tungsten for the material between the lines. As you crowd more and more smaller transistors on a chip, you might want more speed, but what you also get is increased resistance, higher capacitance, and unwanted heat generation.” Weihs is investigating a switch from aluminum to copper, but that, too, has its problems.

“Copper has a lower natural resistivity,” Weihs says, “but it tends to diffuse rapidly into the silicon substrate, destroying the junctions. If we want to use copper, we need a barrier layer that encapsulates it. The question now becomes, what is the best material to use?” Chrome and carbon are possibilities since they do not mix with copper, and Weihs continues his research on these two materials with other department faculty.

Weihs is also interested in the growing field of micromechanical and nanomechanical characterizations. In this area, Weihs and his research group develop new methods for measuring the mechanical properties of very small volumes of material, and they are applying their techniques in novel ways. “Bones and teeth are really composite materials. Our techniques can measure the surface itself, including indentations of a micron or less, and we can also test for hardness. These characterizations can help determine if the bone or tooth is degrading.”

Giving ‘Smart’ Materials a Shock and a Shove
In our society, we test the limits of traditional materials such as steel, tin, and aluminum by demanding goods that are made of lighter, stronger, and more pliable substances. In materials research, developing new, or “smart,” materials continues to mature as a subdiscipline, and characterizing their behavior is critical in determining appropriate applications. In the Materials Center for Excellence, Andrew Douglas, professor of mechanical engineering, and graduate student Steve Marra examine the piezoelectric behaviors of a particular class of new materials, with assistance on experimental issues from K.T. Ramesh, associate professor of mechanical engineering. The term “piezoelectric” is derived from the Greek word piezein, to press.

“We want to understand the mechanical properties of ceramic/polymer composites,” says Douglas. “Some materials, such as the ones we study, exhibit piezoelectricity in that the application of a mechanical pressure generates an electric current. We can study the interaction between the electrical and mechanical properties and even measure the voltage generated.” There is also a reverse piezoelectric effect—passing an AC current through certain crystals will create a regular mechanical vibration. Quartz watches and timers are perhaps the most popular applications of the reverse piezoelectric property.

With their work funded by the Army Research Laboratory, Douglas and Marra get their samples from Professor Dilip Das Gupta, of the University of Wales in Bangor. The samples, which resemble sticky packing tape, have a polymer base to which a ceramic powder has been added. The polymer’s ceramic “chunks” are between five and ten microns in size and are close enough so that the electrical charge passes from one chunk to the next. “The potential applications for these materials are numerous,” Douglas explains, “particularly for use as sensors where a ceramic material won’t work.” For example, a regular rotor blade on a helicopter could become “smart.” Making a ceramic/polymer composite part of the blade itself could provide the operator with data about the blade’s operation through the electric current generated by the composite. Though highly speculative, another potential use of such materials is in the development of artificial muscle.

Phosphors for the Home and Office
One day not too far in the future, our television screens and computer monitors will be as flat as the proverbial pancake, and they will include full color capabilities. How will this be possible? To paraphrase perhaps The Graduate’s most remembered line, “Just one word...phosphors.” Graduate student Eva Wong works with Professor Peter Searson to develop phosphors for flat panel displays. Phosphors are materials that luminesce, and they have attracted widespread attention among scientists and manufacturers for their potential applications in many areas, including high definition television.

With funding from the Microelectronics Research Collaborative Program by the Whiting School and the Army Research Laboratory, Wong studies the characteristics of zinc oxide as a phosphor. First, she grows zinc oxide particles on the nanoscale in an alcohol-based solution. Using ultraviolet light in absorption techniques, Wong then calculates the size of the particles. She can also “age” the particles and study them further by changing the temperature and other factors in the solution. “Traditionally,” Wong explains, “zinc oxide pieces were ground to a fine powder to get the grains small enough. But the grinding process could introduce defects and also contamination. Growing the particles means better control and an improved product.” A higher grade of phosphors can be excited at a lower voltage, critical to designing enhanced portable display devices.

The next step for Wong is to layer the phosphor on a substrate. “There are many phosphors, and depending on the chemical composition, each emits a single color such as blue, green, red, or white when it luminesces. If you layer several phosphors, each a different color, then you have the potential for a full color display device.” Zinc oxide is one of several green phosphors.

Wong, who is beginning her second year of doctoral research, received a bachelor’s degree in materials science from the University of California, Berkeley, and a master’s degree in ceramic engineering from the University of California, Los Angeles. Between her undergraduate and graduate studies, Wong worked at Solar Turbines and the Quantum Group, both San Diego-based companies.

Established 1983
Materials science was first part of the Department of Mechanics, established in 1972, then as part of civil engineering, until it took its present form in 1983.

Phone 410-516-8145

Email materials@jhu.edu

WWW http://www.jhu.edu/~matsci/

Students
1995-96 Academic Year
Graduate: 50
Undergraduate: 24

Faculty and Researchers
James W. Wagner, Chair
Robert C. Cammarata
Michael J. Ehrlich
Robert E. Green, Jr.
John Hoffman
Emanuel Horowitz
Todd Hufnagel
Jerome Kruger
Allan Melmed
Dennis C. Nagle
Theodore O. Poehler
Moshe Rosen
Peter C. Searson
James B. Spicer
Timothy Weihs
John M. Winter, Jr.

Research Areas
Ceramics
Composites
Electrochemistry
Electronic Materials
Materials Degradation
Metallic Materials
Nanostructured Materials
Nondestructive Evaluation
Polymers
Surface Science
Thin Films

Materials Science and Engineering Materials That Can Take the Heat Giving ‘Smart’ Materials a Shock and a Shove Phosphors for the Home and Office Department Facts

Materials Science and Engineering
Materials That Can Take the Heat
Giving ‘Smart’ Materials a Shock and a Shove
Phosphors for the Home and Office
Department Facts