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ramic and the metal and a close match of the coefficient of thermal expansion (CTE) between materials. The CTE defines how an object’s size changes as temperatures change. A glass-ceramic with crystalline phases formed inside the original glass in- creases the CTE to better match the metal housing and reduce thermal stresses.
High-Temperature Processing Since bonded glass-metals must
be processed at very high tempera- tures, “We need to manage the ther- mal mismatch very carefully to make sure during any stage in the sealing process there’s no tensile stress or tension on the glass that will cause a crack or unrecoverable separation from the metal housing,” Dai said. A seal that’s strong at high tem-
peratures and pressures also has po- tential industrial uses, such as in fu- el cells and aerospace or defense ap- plications that operate in extreme environments. Pure glass shrinks less in high
temperatures than metal does. The mismatch causes metal to crimp, com- pressing the seal. That has both ad- vantages and disadvantages. “The good thing is you don’t have to have very good bonding because there’s a lot of compression; the downside is that there could be too much compression, which could crack the glass over time,” Dai said. His team looked at making a chemical bond between metal and glass-ceramics, without adding steps to production, by establishing an in- terfacial bonding layer, a bridge mate- rial that bonds to both steel and glass. “It’s very difficult because these are two very dissimilar materials, a piece of steel and a piece of glass-ceramic. They hardly share anything,” Dai ex- plained. Glass-to-metal seals are pro -
cessed in an inert atmosphere devoid of oxygen because metal grabs oxygen from the atmosphere, leading to oxida- tion and rust. But the process contains
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an inherent contradiction: A metal bond to a glass-ceramic requires an ox- ide, so the interfacial bonding layer is really an interfacial oxide layer. “That’s the fundamental chal-
lenge, how do we do that?” Dai said. Some processes pre-oxidize the met- al, but Sandia wanted to avoid that extra step.
Modified Glass-Ceramic with Oxidant
Dai’s thermodynamic approach
modified, or doped, the glass-ceramic sealant with an oxidant. That oxi- dant, serving as a sacrificial metal oxide, decomposes and migrates at high temperatures, which oxidizes the metal chromium in the stainless steel. The chromium oxide bond formed at the glass-ceramic and met- al interface results in hermetic seals. The team made 24 potential
modified glass-ceramic compositions using a variety of metal oxides that were non-toxic and reasonably easy to handle, such as cobalt oxide. “Most
Strong bonds between materials for airtight, or
hermetic, seals are crucial, and Sandia National
Laboratories continues to advance how it’s done.
of the work is really saying, ‘OK, how many metals from the periodic table can we use and when we dope our glass with these sacrificial metal ox- ides, what quantity do we need to dope it?’” Dai said. Researchers want the doped ce-
ramic-glass material to give up oxygen at the interface, not at the surface of glass-ceramics. “The idea is giving up oxygen in the right place. That’s kind of a fine line that has to do with the properties of the materials and the way you process them,” Dai said. The team identified two modi- fied glass-ceramic compositions that
Continued on page 46 Contents
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May, 2016
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