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Process BasicsIn conventional heating, conduction, convection and/or radiation mechanisms deliver thermal energy to the surface of the sample. The conduction mechanism then transfers the thermal energy into the bulk of the sample. In microwave heating, the heating mechanism is different. Molecular interaction with the electromagnetic field helps deliver microwave energy directly and simultaneously into the sample's whole bulk.
During microwave heating, electromagnetic energy is converted to thermal energy within the sample; the mechanism is therefore essentially one of energy conversion, not just heat transfer. When microwaves penetrate and are converted to thermal energy within the material, heat energy is generated throughout the volume of the material; this results in volumetric heating and rapid, uniform heat-up of the bulk material.
The thermal gradient profile in a conventionally heated sample could be thought of as the reverse of that in microwave heating, in that the center could be slightly less hot than the outside because the thermal energy flows inward. In microwave heating, however, the center may be slightly hotter due to the internal energy conversion mechanisms, coupled with heat losses from the surfaces.
If necessary, additional external susceptors placed strategically nearby could help neutralize any such thermal non-uniformity to achieve uniform and rapid temperature ramp-up in suitable material phases. It has been reported that, in addition to the radiant heat produced by nearby external microwave susceptors, a direct microwave effect on the sample occurs that could increase the sintering kinetics by 1-2 orders of magnitude. This could result in increased heating rates, lowered sintering temperatures, reduced sintering times and improved mechanical properties.
Since microwaves can lead to energy transfer at the molecular level and rapidly generate heat throughout the volume of the material, the potential exists to reduce overall processing time with enhanced product quality in the case of predominantly thermal processes such as sintering. When microwaves pass through a composite/mixture in which the constituents have different dielectric properties, they will selectively couple and heat up the higher loss tangent constituent.
This effect can facilitate the selective heating of materials and could enable rapid "anisothermal heating reactions" to occur. These microwave-enhanced rapid reaction processes have been found to have interesting applications in the sold-state chemical synthesis of materials with specific characteristics that are needed for various commercial/industrial applications.
Microwave processing offers several advantages over conventional methods in various application areas. These include significant enhancements in reaction and diffusion kinetics, much shorter process cycle times, substantial energy savings, finer microstructures that result in better performance products, eco-friendly processing, and more.
In microwave-enhanced synthesis and sintering, enhancements in reaction kinetics and materials diffusion have been reported. The general observation has been that microwave-sintered products have finer microstructures and certain other features that result in considerable improvements in mechanical properties, leading to overall improvement in the quality and performance of the processed parts.
Various ceramic materials and composites have been produced to advantage through microwave processing techniques. This article deals with one of microwave sintering's real-world application products: zirconia-based dental restorations.
Material CharacteristicsStabilized zirconia was introduced as "ceramic steel" in 1975 and was incorporated in biomedical applications as an alternative to brittle alumina. Since then, stabilized zirconia has been used in applications ranging from replacement femoral head implants to dental implants and restorations.
The addition of 2-7 vol% yttria improves the strength of zirconia by changing the crystalline structure from monoclinic to tetragonal. While this provides increased fracture resistance, it can also make the material susceptible to an ageing effect called "low-temperature hydrothermal degradation." Yttria can react with water vapor to form a hydroxide, which can, in turn, produce instability of the tetragonal phase and lead to the material reverting back to its original monoclinic structure.
Such a structural transformation would be associated with an approximately 4.5% volume expansion, which can lead to the formation of microcracks and decreased grain adhesion strength in the material, and thus a loss of structural integrity and possible component failure. This process may take a few decades if the component is exposed to typical ambient temperatures, but it could be accelerated to a few hours if the exposure temperatures are in the 300-400°C range. According to research investigations, this hydrothermal degradation can be reduced or even eliminated by:
- Adding another material (e.g., alumina or ceria) in small quantities to the 3 vol% yttria-stabilized zirconia (3Y-TZP) composition
- Controlling the microstructure during sintering
- Decreasing the sintered grain size to ~ 0.42 microns
Microwave Sintering AdvantagesIt is in this context that the microwave sintering of zirconia-based dental restorations assumes great practical significance. As mentioned earlier, microwave sintering could lead to finer microstructures in ceramics. Such characteristics in microwave-sintered, zirconia-based dental restorations could not only improve the strength of the material but, if the resulting grain size is comparable or less than the reported critical value of 0.42 microns, may also help reduce or eliminate the hydrothermal aging effect. The fine microstructure may provide an additional aesthetic advantage in greater translucency of the finished restoration, which helps achieve a more natural appearance for the end user (patient).
Comparative analytical studies were undertaken of several different zirconia-based material samples sintered through conventional and microwave techniques. A 3Y-TZP composition was used for a detailed scientific study ("Flexure Strength and Hydrothermal Degradation of Yttria-Stabilized Zirconia: Microwave vs. Conventional Sintering") by Arizona State University researchers Professor Pedro Peralta and Kirk Wheeler of the Department of Mechanical and Aerospace Engineering. The resulting samples were tested according to standards established by the American Dental Association. Post-sintered flexure strengths were measured and correlated with the sintering method and resulting microstructure.
It was found that microwave-sintered samples had increased average flexure strength compared to identically prepared samples that were sintered in a conventional furnace. This was attributed to a sintering process that facilitates rapid densification due to enhanced sintering kinetics, as well as substantially smaller grain sized microstructure, through microwave processing.
Long-term degradation effects were also studied through an accelerated ageing test that was performed to simulate the long-term degradation of 3Y-TZP in a humid environment. Samples sintered by microwave and conventional techniques were tested in a steam environment at approximately 125°C and 200 kPa pressure for 75 hours.
Flexure strength decreased by around 43% for conventionally sintered samples, while the decrease of flexural strength was only about 14% for microwave-sintered samples. The decreased degradation of the microwave-sintered material was also attributed to much smaller grain size; the ageing effect is decreased with decreasing grain size and is expected to be eliminated for grain size of around 0.43 microns. Though the grain size had not decreased to this critical size in the current microwave sintering trials, a decreased level of hydrothermal degradation was nevertheless observed. Additional improvements to the sintered grain size may be made by using starting materials of specific characteristics and using suitably modified sintering profiles.
Commercial SystemsThe results of the scientific study clearly reveal the advantages of microwave sintering over conventional sintering for dental zirconia-based materials. To take advantage of these findings on the commercial level, advanced microwave sintering furnaces specially designed for the dental restorations industry are now available in the U.S.
One such furnace, the HAMiLab ADS (private labeled as the MicroSinterWave A1614) microwave batch furnace, is specifically designed to meet the low-volume production throughput requirements of individual dental labs. The ADS system is an air-cooled microwave furnace and offers an effective uniform heating volume of approximately 100 x 100 x 60 mm within an applicator of around 265 x 265 x 315 mm.
These ADS systems, with a maximum microwave power output of 1.1 kW, feature built-in computer capabilities for temperature programming and control in the range 250-1650°C. Temperature measurement and control is achieved through an IR pyrometer, while a touch-pad control panel provides overall system programming and control.
The system includes factory preset programs for automatic mode operation that would enable zirconia-based material sintering runs to be completed in about 90 minutes. Various operator-defined temperature ramp-up rates, temperature hold stages (up to about 1550°C) and hold times may also be programmed, if necessary. Test samples (sintered under conditions similar to those used in the previously mentioned scientific study) revealed increased flexural strength and decreased ageing effects in microwave-sintered samples.
For dental labs and establishments with higher volume production throughput requirements, the Spheric/SynoTherm AMPS continuous microwave furnace could be a viable option. Efficient and completely automated, this system is being used in high-temperature sintering, high-temperature reactions, and high-temperature heat treatment of various ceramic and non-ceramic parts and compositions. Having an upper sinter zone operating limit of 1550°C, the furnace can be configured for sintering zirconia-based dental restoration parts as well.
The AMPS-9 continuous system is a pusher-type furnace with rectangular crucibles of approximately 150 x 100 x 60 mm and a lateral system speed of 15-600 mm/h. The system, which is designed to provide microwave power output of 0.9-9 kW or higher (up to 100 kW, depending on customer needs and specifications), features built-in computer capabilities for temperature programming and control in the range of 100-1550°C. (See the specifications sidebar for additional details.)
The system can be programmed for operation with various temperature ramp-up rates to predetermined temperature hold stages (up to around 1550°C) and hold times. Predetermined stages provide the system with the versatility required to develop processing regimes and profiles of relevance to various materials in general and zirconia-based dental restoration parts, in particular. Preliminary trials on an AMPS continuous microwave furnace have shown encouraging results for the successful industrial-scale, large-throughput microwave sintering of zirconia-based dental restoration products.
Scale-Up from Batch to ProductionExpanding from ADS batch microwave systems to AMPS continuous microwave systems would enable larger throughputs for dental establishments that may have larger volume production needs. Once the best temperature-time profile for the zirconia-based composition of interest is established through ADS batch processing trials, this knowledge can be adapted to develop the AMPS continuous processing routine for larger production throughputs. This would mean transfer of the preferred processing profile in time from the ADS batch microwave furnace to a processing profile in space for the AMPS continuous microwave furnace.
A schematic of the AMPS-9 continuous microwave furnace is shown in the middle of Figure 1. A total of 28 pusher boats are in the processing area at any time during the continuous operation. The furnace processing area is divided into three sections: preheat zone, sinter-heat zone and cooling zone. Four temperature monitoring points (T1, T2, T3 and T4) are included. Of these, the first three are active and provide feedback for microwave power regulation (and therefore temperature) at their locations. T1 can reach a maximum of 700°C, whereas T2 and T3 can reach around 1550°C. Each monitoring point can be used to program predetermined set temperatures in their respective locations.
Shown above the AMPS schematic is the best time-temperature processing profile (ramp rates, holding temperature and holding time) obtained from successful sintering runs of zirconia-based dental restorations in an ADS microwave batch furnace. The AMPS system is then programmed for suitable set temperatures at T1, T2, and T3 to achieve a profile in the AMPS-9 system similar to that shown below the AMPS schematic.
Both T2 and T3 are programmed to reach the required sintering temperature so there is a maximum temperature plateau in the T2-T3 section. A boat push-through rate is then chosen for the AMPS system that, when multiplied by the number of boats at the required temperature in the sinter-heat zone, corresponds to the optimized hold/soak time of the ADS microwave batch processing profile.
This enables each pusher boat to traverse the constant sintering temperature plateau region (or be in the sintering temperature region) for a period corresponding to the hold/soak period in the ADS batch microwave furnace sintering profile. Thus, a preliminary "temperature variation in time" profile can be used to obtain a corresponding "temperature variation in space (distance)" profile for optimum AMPS continuous microwave furnace operation.
Such a strategy has been successfully implemented for the processing of certain types of ceramic materials in an AMPS continuous microwave furnace currently operating in the U.S. The results show that the broad, major challenges of scale-up from limited throughput batch production to larger throughput continuous production levels can be met using the advanced microwave continuous pusher furnace.
The AMPS' advanced capabilities, including programming and control capabilities for time-temperature, lateral material movement rate, and microwave power input-combined with the system's continuous feed-through operation mode-help achieve the commercial-level, large-throughput microwave sintering of various dental restoration components.
ConclusionsThe microwave sintering of ceramics offers several advantages over conventional sintering methods. A detailed scientific study carried out by university researchers revealed that samples sintered in a suitable microwave furnace 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 the enhanced sintering kinetics produced by the microwaves.
Samples tested in a steam environment at around 125°C and 200 kPa pressure for 75 hours showed a reduction of flexure strength by about 43% for conventionally sintered samples and only 14% for microwave-sintered samples. It was estimated that this corresponded to about 10 years of simulated testing in the mouth, after which microwave-sintered zirconia does not deteriorate significantly and is nearly 50% stronger than traditionally sintered zirconia.
Industrial microwave furnaces are now available to exploit the advantages of microwave sintering for zirconia-based dental restorations. A batch-type ADS microwave sintering furnace has been specially designed for relatively smaller throughput dental labs, while an AMPS continuous microwave pusher furnace is available for larger throughput requirements. The processing profiles obtained in a microwave batch furnace can be a helpful step in the subsequent scaling-up of the process for larger production throughput with the AMPS continuous microwave furnace.
For more information, contact Spheric Technologies Inc. at 4708 E. Van Buren St., Phoenix, AZ 85008; call (602) 218-9292; fax (602) 218-9299; e-mail firstname.lastname@example.org; or visit the website at www.spherictechnologies.com.