Each of the two X-ray techniques, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), reveals different types of information about the nature of materials. "Each technique alone will only tell you one thing, but together they can give some special results," said William A. Brantley, professor of restorative dentistry, prosthodontics and endodontics at Ohio State.
Along with John Mitchell, senior electron microscopist, and Efstratios Papazoglou, clinical assistant professor of restorative dentistry, prosthodontics and endodontics, Brantley explained that a combination of XPS and XRD revealed new information about the complex near-surface structures of the oxidized palladium alloys that dentists commonly use for conventional restorations and implant-supported crowns and bridges. Such restorations typically consist of a porcelain crown fused to a metal base.
A description of this research, which was funded by the National Institutes of Health, appeared in a recent issue of the Journal of Materials Engineering and Performance. The two research papers were co-authored by the Ohio State researchers and their partners at Material Interface, Inc. and the University of Wisconsin-Milwaukee.
"As it is now, the palladium alloys used in dentistry are adequate, but we're trying to learn more about their structure, the origins of their mechanical properties, and the way they adhere to dental porcelain," said Brantley. "Nobody has ever tried to optimize the palladium alloy," he continued. "The dental manufacturers don't have the time or resources to investigate these materials on a basic level."
Palladium dental alloys are stronger and stiffer than gold alloys, so they should last longer. They also don't permanently deform or break as easily under chewing forces, which for human molars can measure as high as 150 pounds. Ohio State currently utilizes a palladium alloy in its dental clinic for restorations. Brantley said patients can't tell the difference between a restoration made with palladium alloy versus one made with a traditional gold alloy.
Makers of conventional restorations first fashion a metal framework and then heat it at high temperatures to allow the atoms on the surface to bind with oxygen. The layer of oxide on the surface of the alloy helps the framework stick to the dental porcelain, the portion of the restoration that looks like a normal tooth. They then fire the porcelain onto the metal base.
Brantley said he and his colleagues applied the dual X-ray technique to this critical oxide layer because scientists know very little about its structure, especially for the complex palladium dental alloys which contain many different elements.
The first X-ray technique, XPS, reveals the oxidation state of atoms on the surface -- that is, the number of electrons that the atoms gain or lose when they form a chemical bond with another atom. XPS can also determine the concentration of atoms belonging to different oxidation states.
The second technique, XRD, takes advantage of the fact that the atoms of alloys arrange themselves into crystal lattices. If X-rays of the proper wavelength enter an alloy at just the right angles, the X-ray beam is reflected by the atoms. Each region of unique composition and structure in a crystalline material has its own characteristic set of X-ray reflection peaks, so scientists can use the peaks like a fingerprint to identify the regions within the material.
The researchers examined two different oxidized palladium alloys, and XRD revealed that the position of the peaks didn't match those previously published in the scientific literature. Then, when the researchers applied the XPS technique to the same alloys, they saw something that hadn't been seen before -- that some of the palladium oxide had chemically bound with water, most likely from the residual atmosphere found in the dental vacuum furnace used to fire the porcelain.
"Only through the use of XPS could the presence of this important phase be clearly identified," said Brantley. "Once the XPS information was available, we could interpret the XRD results to mean that the hydrated palladium oxide wasn't at equilibrium, or was experiencing stress in the oxidized layer. That's what changed the position of the XRD peaks."
Another novel result from the use of XPS was the researchers' discovery within the oxide layer of microcrystals of gold and copper, which also altered the XPS peak energies but were too small to detect with traditional XRD.
The dual X-ray technique cleared up some mysteries concerning the performance of palladium dental alloys. For instance, a previous study led by Papazoglou found that palladium-copper-gallium alloys adhered to porcelain much better than palladium-gallium alloys. An examination of both alloys in this current study revealed why.
In general, oxide layers on alloys grow in one of two ways: down into the surface of the alloy, or out into the air. XPS revealed that strong oxide layers on palladium-copper-gallium alloy grow out into the air, whereas the weaker oxide layers on the palladium-gallium alloys grow down into the alloy surface.
"We think that explains why the porcelain adherence is superior for the palladium-copper-gallium alloys," said Brantley. "We can use this information to design new alloys that will bond to porcelain even better."
Contact: William A. Brantley, (614) 292-0773;
Brantley.1@osu.edu
Written by Pam Frost, (614) 292-9475;
Frost.18@osu.edu
Journal
Journal of Materials Engineering and Performance