David J. Green, professor of ceramic science and engineering, worked
with others to develop a theoretical approach to designing strengthened glass.
Photo: Greg Grieco
David J. Green, professor of ceramic science and engineering, worked
with others to develop a theoretical approach to designing strengthened
glass.
Photo: Greg Grieco
By A'ndrea Elyse Messer
Public Information
Few things are as fragile as glass, and if a researcher has his way, some types of glass will be less fragile.
"Chemical- and heat-tempered glasses have been around for a long time," said David J. Green, professor of ceramic science and engineering. "These glasses can withstand more stress before breaking than untreated glass, but when they break, they usually break catastrophically."
Another problem with chemical- and heat-tempered glass is that while each individual piece of glass becomes stronger, the variability of strength between pieces of glass increases dramatically. Engineers choosing glass for specific purposes must account for this wider range of strengths.
Working with R. Tandon of Caterpillar Inc. in Peoria, Illinois, and V.M. Sglavo of the University of Trento, Italy, Green developed a theoretical approach to designing strengthened glass.
Conventional tempering of glass alters the outer surface of the glass so that it is under compression. Glass under compression can withstand higher levels of stress before reaching the failure point.
"Rather than simply altering the outside layer of glass, we would like to engineer the glass so that it has a specific compression profile making the final product stronger and less variable," said Green.
The researchers tested their theory using the chemical tempering process on sodium aluminosilicate glass, but believe that they could adapt the process to other tempering processes and other materials.
In chemical tempering, potassium atoms are often used to replace some of the sodium atoms near the surface. These potassium atoms are slightly larger than the sodium atoms and they compress the layer in which they are substituted by crowding the other atoms. Chemical tempering usually occurs in the outer millimeter of the pane of glass.
"If we place the maximum compression layer beneath the surface, when cracks propagate from the flaws on the surface, they reach the layer and stop," said Green.
The researchers created these internal compressed layers by subjecting the glass to chemical processing where potassium substituted for sodium, but then exchanged some of the potassium near the surface back to sodium. This created glass with an untempered surface, but with a tempered, compressed layer below.
"Unexpectedly, glass made in this way exhibits multiple cracking," said Green. "Unlike untreated glass or conventionally tempered glass where a crack that begins progresses rapidly to catastrophic failure, small cracks begin to form in the untempered layer and then the cracks are arrested by the compressed layer."
Many cracks may form before the ultimate crack that propagates through the compressed layer and shatters the glass. This surface crazing can be used as a warning that the glass is approaching its breaking point and needs to be replaced. Creating glass that will only break at a certain, predetermined stress level also may be possible.
"The strength range of a batch of conventionally tempered glass may be as broad as 25 percent on either side of the average strength," Green said. "However, the specially designed glass we are looking at has a range of only 6 percent on either side of the average." This smaller range provides more consistency when manufacturing the glass.
Chemically tempered glass is used in eyeglasses and sunglasses and thermally tempered glass is used in automobile windshields. This new tempering method could allow thinner glass to be used in such things as photocopying machines, scanners and electronic displays that would make them stronger and lighter. Eventually, glasses could be designed with specific strengths and a higher reliability.
With a five-year federal grant, a team of researchers in the College of Medicine will undertake three projects that focus on distinct aspects of iron metabolism in the human body.
The projects are expected to lead to a greater understanding of imbalances in iron stores that can occur in the human body, which can cause some of the most prevalent and debilitating diseases known to modern medicine.
Among these diseases are anemia, which results from too little iron and results in weakness and lethargy, and hemochromatosis, which results from too much iron and can lead to heart problems, diabetes and liver cancer.
The team will be led by Michael J. Chorney, associate professor of microbiology and immunology and pediatrics. The team's projects will be funded with a five-year program project grant from the National Institute of Diabetes and Digestive and Kidney Diseases, part of the National Institutes of Health.
One of the projects will be led by Chorney. The others will be led by James R. Connor, professor of neuroscience and anatomy, and Harriet C. Isom, professor of microbiology and immunology and pathology.
Thermal storage, which boomed in the 1980s and then went bust as an energy- and cost-saving air conditioning approach in the 1990s, is actually the best choice today for some large system applications such as district cooling or industrial process cooling.
William Bahnfleth, assistant professor of architectural engineering, and Amy Musser, former Penn State student and now a postdoctoral research associate at National Institute of Standards and Technology, said the key to appropriate use of thermal storage is selecting applications that don't depend on electric company rebates or rate incentives for their cost competitiveness.
Bahnfleth, a consultant on a recently completed thermal storage project at the University of Western Australia, is currently working on four million-gallon thermal storage projects at two American universities.
Stratified chilled water storage systems depend on a storage tank where chilled water can be stored during off-peak energy use periods, usually at night, and then used for cooling during peak demand periods. Chilled water storage systems can become lower in capital cost than equivalent mechanical refrigeration capacity, while at the same time reducing operating costs, he said.