asu-asbstract-229wA new study from the University of Arizona compared the strength of microwave versus conventional sintering of zirconia, and found that while all zirconia is marginally harder (5-20%) as a result of microwave sintering, the most dramatic results occur over time when traditionally sintered zirconia deteriorates in the mouth. This study is the most comprehensive independent analysis of zirconia sintering methods to date, and is available to our clients as a PDF via our online request form.

The study, entitiled "Flexure Strength and Hydrothermal Degradation of Yttria-Stabilized Zirconia: Microwave vs. Conventional Sintering," written by Professor Pedro Peralta and Kirk Wheeler of the Department of Mechanical and Aerospace Engineering at Arizona State University came to the following conclusions:

  1. "Samples sintered in the microwave oven had an increased flexure strength compared to samples sintered in a conventional furnace. This was attributed to a reduced grain size and a sintering process that encourages rapid densification due to enhanced sintering kinetics produced by the microwaves."

  2. "Samples were tested in a steam environment at ~125

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ceramic-industry-ride-the-waveOur microSinterWave continuous oven technology was featured in a 2008 article in Ceramic Industry Magazine. The article indicated that high temperature microwave sintering can be faster, greener and cheaper than conventional heating technologies, and that in addition to substantial economic advantages of faster processing times and less energy usage, the final products have comparable or even better mechanical properties than those which are traditionally heated.

The difference between an industrial microwave and a cooking-type home microwave is all about the temperatures involved, and the ability of consumer and even commercial kitchen microwave components to withstand temperatures above those produced by traditional kitchen ovens (about 500 degrees fahrenheit). With the new sophisticated breed of high temperature furnaces such as the microSinterWave line of products, industrial components can handle heat as high as 1600 degrees centigrade (3000 degrees fahrenheit).

The report indicated that many companies are exploring the use of microwaves to sinter their parts because of their "green" nature. For most applications, microwave furnaces focus their energy directly on the parts to be sintered, not on the atmostphere and the surrounding walls. Energy-efficient microwave furnaces produce a significantly smaller carbon footprint compared to electric- or gas-powered air chamber furnaces, producing and less pollutants and reducing the overall cost and carbon-footprint of each finished product that is sintered in these ovens.

Microwave energy operates differently on different types of matierals. Zirconia, for instance, does not couple with microwave energy at room temperature, so other companies have employed either microwave assisted technology or a hybrid design, where air-chamber heating is used first, followed by full microwave finishing. With our smaller tabletop microSinterWave ovens, we use low-tempature microwave energy to heat up susceptors embedded in the crucible which accelerates the heating at lower temperatures, thus kickstarting the zirconia to a higher temperature where it becomes more susceptable and can bond directly with the microwave energy. While hybrid designs have been used to cut processing times and energy costs by 50%, our 100% microwave solution is significantly faster and greener, resulting in 70-90% quicker processing times and 80% energy savings, compared to traditional furnaces.

Using a microwave for sintering can also produce improved product quality with finer finished grain sizes, higher density, better corrosion resistance, and superior bending strength. Initially, we are seeing the largest unit application of this technology in bio-ceramics, including dental laboratories which use the technology to sinter Zirconia copings and frameworks, but this technology can also be used to sinter any of the high-temperature ceramics or parts made of "hard metals." Because of the improved matierial properties, quicker production times and lower energy costs, in the past few years, we have seen high temperature microwave technology employed at a large scale in manufacture and finishing of advanced ceramic-carbide wear parts, electro-ceramics, and even moving to new industries, such as metallurgy, material synthesis, and high temperature chemistry applications. We have even seen some pretty amazing work on using microwave sintered capsules to seal in toxic and radioactive materials for waste remediation.

Microwave energy is not only the technology of tomorrow, but at least for the dental lab industry, which can produce massive zirconia production from a single table-top unit, it is also the leading technology of today. With a smaller physical footprint than traditional furnaces and a substantially smaller carbon footprint, microwave furnace technology offers lower costs per unit for zirconia copings and framework, making it a greener and friendlier technology for any company looking to replace their existing equipment or to expand their production.

microwave200The sintering process required to harden zirconia requires much higher temperatures than are required to harden typical ceramics. Laboratories often have mixed results from their sintering effort depending on the systems and technologies employed to do the sintering. Unlike a traditional dental radiant heat furnace, a microwave oven sinters copings and frameworks through direct application of energy on the dental ceramics, and not solely by heating the surrounding materials and air chamber. Due to significant speed and quality advances, we believe that in the future, all sintering ovens will be microwave based.

There are many reasons to use a microwave sintering furnace, chief among them being to create stronger dental substructures in a quicker turn-around time while saving energy costs. Because a traditional oven heats the substructures from the outside inward, uniformity of the crystalline microstructures cannot be fully realized via radiant sintering.

Below is a list of some of the components of a microwave dental furnace and the role that these components play in the sintering process:

Microwave Transparent Crucible - In a mircowave furnace, the crucible is made of a light insulating material that is transparent to microwaves allowing the waves to pass through unimpeded to the zirconia. Unlike traditional furnaces which have hardened ceramic crucibles, the material used to create our microwave crucible must be low density and have very low dielectric loss. Our crucible design is also speckled with silicon carbide granules which act as a susceptor, helping to rapidly increase the heat inside the crucible. The purpose of the crucible is to evenly and inwardly focus the heat energy created by microwave and the susceptors so that the fastest ramp-up times can be achieved, and that energy will not be wasted heating the entire chamber.

Susceptor Plate - Zirconia and most other ceramics do not absorb (couple with) microwaves at room temperature, so a susceptor is used to heat the zirconia to the point where it absorbs microwaves. For zirconia, at about 1000

goldversuszirconiaDental ceramics such as Zirconia offer a desirable alternative to metals as a dental restorative material due to its aesthetic qualities, durability and biocompatibility.

Sintering ceramic substructures in a conventional oven, however, is a lengthly procedure in which a large oven must be heated to a very high temperature and then slowly cooled to prevent cracking of the ceramic material. Dental laboratories, overloaded with requests, usually take seven or more days to process the permanent dental restoration with a conventional oven. In the meantime the patient is fitted with a temporary crown. This time consuming process results in loss of work productivity, involves increased riak of morbidity to patients from problems with a temporary crown and occasional problems with improperly fitted permanent crowns.

A microwave furnace can sinter dental ceramics within 1/12th the time of a conventional oven. Microwave has the added advantage of producing better mechanical strength and resistance to low temperature degradation. Microwave is the best choice for sintering dental ceramics.

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