EP1665889A4 - Procede de traitement micro-onde de ceramiques et systeme de chauffage hybride micro-onde associe - Google Patents

Procede de traitement micro-onde de ceramiques et systeme de chauffage hybride micro-onde associe

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Publication number
EP1665889A4
EP1665889A4 EP04782755A EP04782755A EP1665889A4 EP 1665889 A4 EP1665889 A4 EP 1665889A4 EP 04782755 A EP04782755 A EP 04782755A EP 04782755 A EP04782755 A EP 04782755A EP 1665889 A4 EP1665889 A4 EP 1665889A4
Authority
EP
European Patent Office
Prior art keywords
microwave
temperature
susceptor
sintered
sintering
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04782755A
Other languages
German (de)
English (en)
Other versions
EP1665889A2 (fr
Inventor
Subrata Saha
Vc Ram Mohan
Gary E Delregno
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Alfred University
Original Assignee
Alfred University
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Filing date
Publication date
Application filed by Alfred University filed Critical Alfred University
Publication of EP1665889A2 publication Critical patent/EP1665889A2/fr
Publication of EP1665889A4 publication Critical patent/EP1665889A4/fr
Withdrawn legal-status Critical Current

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61C—DENTISTRY; APPARATUS OR METHODS FOR ORAL OR DENTAL HYGIENE
    • A61C13/00—Dental prostheses; Making same
    • A61C13/20—Methods or devices for soldering, casting, moulding or melting
    • A61C13/203—Methods or devices for soldering, casting, moulding or melting using microwave energy
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/80—Apparatus for specific applications
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B2206/00—Aspects relating to heating by electric, magnetic, or electromagnetic fields covered by group H05B6/00
    • H05B2206/04—Heating using microwaves
    • H05B2206/046—Microwave drying of wood, ink, food, ceramic, sintering of ceramic, clothes, hair

Definitions

  • the present invention relates to a method for microwave processing of ceramic materials and a microwave hybrid heating system for microwave sintering ceramic materials.
  • the present invention relates to a method of microwave sintering ceramic materials, including ceramic dental copings and porcelain dental coatings, using a microwave hybrid heating system on site at a Dentist's Office.
  • Dental ceramics can be classified as aesthetic ceramics or non-aesthetic ceramics.
  • Aesthetic ceramics are used in the restoration of missing teeth or tooth structure (where natural smile is desirable), while non-aesthetic ceramics are used as dental implants, dental cements and biologically active ceramics.
  • Aesthetic dental ceramics typically comprise feldspathic minerals, also referred to as dental porcelain, and glass-ceramic materials.
  • Aesthetic dental ceramic materials are used extensively in the Dental Industry to make denture teeth, single-unit crowns, fixed partial dentures and labial veneers, for example. Other common uses of dental ceramics include full coverage crowns, inlays and onlays, porcelain bridges and veneering agents.
  • Restorative dentistry is a branch of dentistry pertaining to oral rehabilitation, or the restoration and maintenance of oral functions, comfort, appearance and health of the patient by the restoration of natural teeth and/or the replacement of missing teeth and contiguous oral and maxillofacial tissues with artificial substitutes (adopted by the ADA on May 1976).
  • Crowns also know as "caps,” are coverings that fit over teeth. As shown in Fig. 1, the crown or cap 100 fits over the supporting tooth structure 200 to give the overall appearance of an unaltered natural tooth. Crowns may be necessary because of damaged or worn out old fillings, fractured, chipped or sensitive teeth.
  • Crowns can also be used to improve the appearance of natural teeth that are malpositioned, malformed or discolored. Crowns are typically composed of porcelain coating layers, which offer a natural look and which can improve the overall smile or simply blend with remaining teeth.
  • An underlying metal or ceramic coping also called a shell
  • dental porcelains are typically applied over a coping, such that the porcelain layers form the outer contour of the dental restorations that are in direct contact with the oral environment and any objects introduced therein, such as adjacent and opposing teeth, food, chewing gum, and the like.
  • a variety of different metal, ceramic and combination crowns are available, depending upon the particular oral rehabilitation situation, aesthetic and economical concerns of the patient.
  • Full Cast Crowns are basically all-metal crowns that are made from high noble or noble metals, generally all gold crowns. Full cast crowns offer excellent biocompatibility, are non-abrasive to opposing natural teeth, require the least damage to the underlying natural tooth (i.e., involve conservative tooth preparation), and are easily adjusted by the dentist both before and after cementation. Disadvantages associated with full cast crowns include the fact that metal teeth may not present an aesthetically pleasing appearance, and medically, some patients have may allergic reaction to the metals. "Porcelain Fused to Metal Crowns” are basically a noble metal substructure or shell layered with dental glass ceramics to provide the desired aesthetic qualities.
  • the advantages include excellent biocompatibility and a generally good color match with respect to the natural tooth that is capped as well as the surrounding natural teeth. Porcelain fused to metal crowns are also typically stronger than other types of crowns. There are some disadvantages associated with these types of crowns, however, including difficulty in adjusting the crown after cementation or placement and the risk that the crown may break due to the brittle nature of the aesthetic dental ceramic layers. Additionally, this type of crown typically requires involves a greater level of tooth preparation than full cast crowns, in that more of the vital tooth structure may be lost in preparation for providing and fitting the crown. "All-Ceramic Crowns" consist of a ceramic core structure, made of, for example, zirconia or alumina that is then layered with dental glass ceramics.
  • all-ceramic crowns include a high level of aesthetic quality, excellent biocompatibility, and all-ceramic crowns can be bonded to the underlying prepared tooth structure for maximum retention.
  • the minor disadvantages of all-ceramic crowns include a risk of breaking, and a greater degree of tooth reduction compared to full cast crowns and porcelain fused to metal crowns.
  • the time required to prepare all- ceramic crowns in a dental lab is fairly extensive. Recently, the trend in the dental industry has been focused toward metal-based crowns. However, in the face of emerging technologies related to dental materials, and also in view of dental patient demands for more aesthetic (non-metallic) teeth restoration options, restorative dentistry has undergone a dramatic change.
  • the process involves multiple visits to the Dentist's office by the patient for tooth preparations, impressions, temporary crowns or restorations, and later, for the final application of the permanent crown.
  • the crowns are individually manufactured at an off-site dental lab, that is, not in-house at the Dentist's Office.
  • the dentist reduces the target tooth uniformly so that the crown, which replaces the lost, damaged, or otherwise removed tooth structure, is not oversized (crowns must have a minimal thickness of 2mm in order to have sufficient retention and strength).
  • the tooth receiving the dental crown is slightly tapered in form so that the crown slips over the tooth and has a snug fit.
  • the dentist casts an impression of the prepared tooth using an impression paste, which is a putty like material. These impressions are then sent to the dental lab along with the appropriate tooth shade that is esthetically acceptable to the patient.
  • the patient is provided with a temporary crown to protect the prepared tooth and to improve the patients comfort, ability to perform perfunctory oral operations, such as eating and drinking, and to compensate for aesthetics (i.e., fill a tooth gap or broken portion) until the permanent crown is ready.
  • the time between the first visit and the final crown fitting and cementation can typically range from 10 days to two weeks.
  • dental technicians at the off-site dental lab use the impressions and pour, for example, Plaster of Paris (e.g., dental stone) to create a plaster cast of the tooth, which is a positive replica of the prepared tooth.
  • Plaster of Paris e.g., dental stone
  • Die-pins are attached to the plaster cast master model of the tooth requiring the crown. The die pins, along with the master model, are carefully examined for any defects.
  • alumina or zirconia slip for the coping is applied over the master mold. Both the mold and the coping are then placed in a conventional sintering oven. The ceramic coping and the mold are then sintered at a temperature of approximately 1450°C.
  • Sintering in a conventional sintering oven typically requires 8 to 12 or even more hours, and may vary significantly between different dental lab facilities.
  • the alumina or zirconia cast coping remains retains the cast shape and size while the mold shrinks. This provides a coping that fits snugly to the master die before firing (which represents the actual tooth onto which the crown will later be fit).
  • the dental technician then applies the desired shade of infiltration glass and fires it. The glass flows between the alumina or zirconia particles and fills the spaces therebetween. This process of infiltration also strengthens the copings.
  • the alumina or zirconia copings are then sandblasted to prepare a proper bonding surface structure in preparation for the application of the porcelain shades.
  • the above steps generally require around 14 hours to fully prepare an infiltrated coping suitable for dental porcelain application.
  • An opaque base which is a combination of body porcelain and opaque porcelain in suitable proportions, is first used to mask the underlying copings.
  • a dentine shade is then applied, which gives the crown a warm glow imitating the natural dentine. Applying these shades prevents the restoration from having an unnaturally opaque look.
  • Two ample coats of dentine shades over the opaque shade will promote natural appearing aesthetics in the ceramic dental restorations.
  • an enamel shade is applied to lend more depth and natural color to the restoration.
  • the enamel shade mimics the natural tooth enamel in appearance.
  • a glaze layer is applied. This layer imparts an overall esthetically pleasing look to the completed restorations and also contributes extra strength to the porcelain restorations.
  • Glaze powders are mixed and manipulated using a dental modeling liquid in order to achieve the proper consistency.
  • the first layer of opaque base is usually baked in a furnace at temperatures of about 860°C to 900°C and at a holding time of about three to five minutes. The same heat treatment procedure is then repeated after the application of each subsequent layer. The whole cycle usually takes approximately 2-3 hours, but may take even longer if the dental technician decides to fire and cool the crown using a slower ramp rate.
  • the crowns are polished using slow speed diamond burs and then finished using a polishing paste. Once the dental lab is completely satisfied with the finished crown, it is sent back to the Dentist to be inserted into the patient's mouth. That is, days, or even weeks from the initial time when the patient first underwent the necessary tooth preparation, the patient returns to the dental office, the temporary crown is removed, and the permanent crown is installed on the prepared tooth with any standard dental cement pertaining to crowns.
  • Other processes for producing full cast crowns and porcelain fused to metal crowns include the lost wax technique and the use of CAD/CAM modeling systems, which are known to those in the dental industry. Still, the copings, whether metal or ceramic, must be sintered in order to impart the desired structural, performance and aesthetic characteristics to the final restoration.
  • the times required to do so are extensive and the entire process is labor intensive, from the steps performed at the dentist's office to the , processes performed at the dental lab and back to the dentist again.
  • the duration of time between the initial delivery of the impression to the dental lab and production of the final sintered crown takes approximately ten to fourteen days. Again, this time span may vary between different dental labs.
  • the sintering or firing cycle of the ceramic coping plays a vital role in making an all-ceramic crown, sintering ceramic copings, and particularly zirconia copings, and the subsequent sintering of the dental glass ceramic itself requires a great deal of time.
  • Zirconia has long been a preferred biomaterial for many reasons, and is particularly suitable for dental ceramic applications.
  • Zirconia zirconium oxide, ZrO 2
  • ZrO 2 zirconium oxide
  • ZrO 2 zirconium oxide
  • One factor that makes stabilized zirconia materials desirable for dental applications is its excellent ability to hinder the progress of cracks, and the process of transformation toughening or strengthening is the key virtue of this material.
  • the physical and mechanical properties of zirconia include a density of 6.1g/cc, a Vickers Hardness (VHN) of 1200, flexural strength greater than 900 MPa, a modulus of elasticity of 210 GPa, a coefficient of thermal expansion (CTE) of 8-10 x 10 " 6 in/in/K for temperatures between 25 and 500°C, a fracture toughness of 9 MPa-m 0,5 and a corrosion resistance less than 10 x 10 "6 g/cm 2 . Additionally, the mechanical properties of densely sintered zirconia exceed the known values of those of many conventionally known dental ceramic materials and actual natural tooth structures.
  • zirconia typically exhibits a bending strength on the order of 900 MPa
  • the bending strength of a glass-infiltrated slip cast alumina e.g., In-Ceram alumina
  • that of industrially fabricated glass-infiltrated alumina e.g., Vita-Celay alumina
  • Leucite reinforced porcelain such as IPS Empress
  • Natural tooth structures have comparably low bending strengths, on the order of 65-75 MPa for enamel and 16-20 MPa for dentine.
  • In-Ceram alumina has a toughness of about 2.4-5 MPa-m 0 5 and the tested toughness of Vita-Celay alumina is about 3.55 MPa-m 0,5 .
  • IPS Empress has a toughness of 1.77 MPa-m 0,5 .
  • Even natural tooth structures have a higher toughness than Omega feldspathic porcelain (0.99 MPa-m 0,5 ), with natural enamel having a toughness of 1 MPa-m 0,5 and natural dentine having a toughness of 2.5 MPa-m 0,5 .
  • Transformation toughening is a mechanism whereby through the appropriate use of additives such as yttrium oxide, zirconia particles can be stabilized with a tetragonal crystal structure at room temperature. Tetragonal crystal structures impart maximum strength to this ceramic material.
  • an external energy source such as a stress at a crack tip
  • toughened zirconia it goes through an instantaneous phase transformation to a monoclinic structure.
  • the surrounding material remains in the tetragonal phase and exerts compressive forces on the monoclinic structure in the crack vicinity, which essentially clamps the cracks shut and restricts further crack propagation.
  • zirconia materials impart excellent toughness and are a superior choice for reliable restorations in restorative dentistry when compared to other potential dental restoration materials.
  • processing parameters e.g., sintering time and temperature
  • Microwave energy offers a fast and effective sintering process that can reduce processing time by as much as 90% and which offers energy savings as a result.
  • the corundum foil can then be used as a base for building a ceramic crown or onlay. Since the same foil that was used to make the original impression becomes part of the crown or onlay, many copying operations are avoided and the precision is retained.
  • the oxide foil is fired at a temperature over 700° C, preferably around 900° C, whereby the oxide recrystallizes to a waterless alpha-corundum with unchanged dimensions.
  • the firing can be made in a conventional muffle oven or with microwave heating.
  • the '912 suggests that microwave heating provides faster and smoother heating concentrated to those parts where some moisture is left, but there is no disclosure or suggestion in the '912 patent as to the specific steps, such as firing parameters, or as to the particular structure of the microwave furnace system itself.
  • Patent No. 6,325,839 to Prasad et al. discloses microwave sintering metal-based dental restoration materials. According to the '839 patent, higher heating rates may be achieved using microwave energy, which reduces the time necessary for sintering the metal-based materials.
  • the '839 patent suggests that is possible to produce high strength metal-based dental restorations at lower temperatures having high hardness and density and small grain size using microwave energy.
  • the metal powder is preferably a high fusing metal and may comprise one or more precious metals, non-precious metals and alloys thereof, and that preferably, the metal powder comprises a non-oxidizing metal.
  • the metal powder is mixed with a binder, and optionally a solvent, each of which are driven off during sintering.
  • the metal may be in the form of a thin metal foil containing one or more of gold, platinum, silver and alloys thereof.
  • the model created is ready for firing.
  • the model is sintered in a microwave apparatus which is similar to a conventional porcelain oven, but which supplies microwave energy to sinter the materials placed therein.
  • the sintering temperature range depends upon the metal or alloy being used, and the sintering temperature is preferably below, but near the melting temperature of the metal/alloy.
  • the sintering range is about 800°C to about 1200°C
  • the sintering time varies depending on the cross-sectional area of the restoration, for example, a dental crown having a very thin cross-section will take less time than a pontic or bridge with a thicker cross-section. While the '839 patent suggests that the sintering time could be as low as about one minute to ten or twenty minutes or as high as one to two hours, there is no disclosure or suggestion in the '839 patent of how to actually achieve such rapid sintering times. The '839 patent also discloses that the sintered metal layer is coated with a ceramic or porcelain material and thereafter sintered in a microwave apparatus or in a conventional porcelain oven to obtain a dental restoration.
  • the '839 patent merely suggests that microwave sintering can be used to produce metal and metal coated dental restorations. That is, there is no disclosure therein of any specific processing parameters or particulars of the MHH system required to sufficiently microwave sinter one or a plurality of such metal or coated metal dental restorations.
  • Dental material researchers agree that new technology permits faster sintering of complex ceramic shapes while maintaining an exact fit and would considerably reduce the manufacturing costs, and would thus be desirable. Further, it would be desirable to be able to sinter many ceramic crowns or restorations simultaneously and to deliver the crowns in a shorter period of time. This particular aspect is of utmost importance in dental treatment protocol.
  • a method for sintering ceramic materials at a sintering temperature greater than room temperature includes the steps of providing a ceramic member to be sintered, wherein the ceramic member comprises a material that does not substantially reflect microwave energy at room temperature and that does not substantially couple to microwave energy until the ceramic material is heated to a microwave coupling-trigger temperature.
  • the method also includes a step of providing a microwave furnace having a magnetron microwave source and a microwave chamber lined with a reflective material and a step of providing a thermal contaimnent box having an inner surface and an outer surface defining a thermal containment chamber.
  • the thermal containment box comprises an insulative material that does not substantially absorb or reflect microwave energy at any temperature lower than a maximum sintering temperature of the ceramic member to be sintered and which has a melting temperature that is greater than a maximum sintering temperature of the ceramic member to be sintered.
  • the method further includes the steps of providing at least one susceptor within the thermal containment chamber, wherein the at least one susceptor comprises a material that directly couples to microwave energy at room temperature substantially immediately, positioning the ceramic member within the thermal containment chamber proximate the susceptor, positioning the thermal containment box within the microwave chamber of the microwave furnace and irradiating the microwave chamber with microwave energy from the magnetron source. The susceptor substantially immediately couples to the microwave energy.
  • Heat is generated by the susceptor and radiated therefrom to heat the thermal containment chamber and the ceramic member positioned therein.
  • the temperature of the ceramic member increases, in response to the radiant heat emitted from the susceptor, to the coupling-trigger temperature of the ceramic member, whereby the ceramic member couples directly to the microwave energy.
  • the method also includes a step of directly sintering the ceramic member with the microwave energy in cooperation with the radiant energy emitted from the susceptor.
  • the susceptor has a microwave reflecting temperature that is greater than or substantially equal to the coupling-trigger temperature of the ceramic member.
  • the ceramic member is heated at least to its microwave coupling-trigger temperature by the radiant heat from the susceptor so that the ceramic member directly couples to the microwave energy before the susceptor reaches the temperature at which it becomes reflective to the microwave energy and ceases to absorb the microwaves and emit radiant heat.
  • the susceptor comprises SiC and the ceramic member comprises zirconia stabilized with 3 mol % yttria.
  • the susceptor comprises at least one primary susceptor and at least one secondary susceptor.
  • each primary susceptor and each secondary susceptor each has a melting temperature that is greater than a maximum sintering temperature of the ceramic member, and the secondary susceptor preferably has a microwave coupling-trigger temperature that is higher than room temperature and lower than the microwave reflecting temperature of the primary susceptor. It is also preferred that the microwave reflecting temperature of the secondary susceptor is greater than the coupling-trigger temperature of the ceramic member. In that manner, the ceramic member is sintered by directly coupling to the microwave energy in cooperation with the radiant heat emitted from the secondary susceptor.
  • the primary susceptor comprises SiC and the secondary susceptor comprises reticulated zirconia.
  • the ceramic members comprise a dental ceramic material, preferably a Y-TZP (yttria tetragonal zirconia polycrystal) ceramic.
  • the ceramic members comprise a sintered dental ceramic material that is coated with one or more layers of a dental enamel material or a glass-ceramic material.
  • a microwave hybrid heating system for sintering one or more ceramic members having a microwave coupling- trigger temperature that is greater than room temperature is provided.
  • the microwave hybrid heating system includes a microwave furnace having a magnetron microwave source and a microwave chamber lined with a reflective material, and a thermal containment box having an inner surface and an outer surface defining a thermal containment chamber.
  • the thermal containment box comprises an insulative material that does not substantially absorb or reflect microwave energy at any temperature less than or substantially equal to a maximum sintering temperature of the ceramic member to be sintered, and has a melting temperature that is greater than the maximum sintering temperature of the ceramic member to be sintered.
  • the microwave hybrid heating system also includes at least one susceptor positioned within the thermal containment chamber.
  • the susceptor comprises a material that directly couples to microwave energy at room temperature substantially immediately.
  • the ceramic members to be sintered are positioned within the thermal containment chamber and the thermal containment box is positioned within the microwave chamber. When microwave energy from the magnetron source irradiates the thermal containment box within the microwave chamber, the susceptor substantially immediately couples to the microwave energy to generate heat.
  • the heat from the susceptor radiates therefrom to heat the thermal containment chamber and the one or more ceramic members positioned therein.
  • the temperature of the ceramic members increases to the microwave coupling-trigger temperature, whereby the ceramic members couple directly to the microwave energy and the ceramic members are sintered by the microwave energy in cooperation with the radiant energy emitted from the susceptor.
  • FIG. 1 is a schematic illustration showing a dental restoration (i.e., crown or cap) positioned on a supporting tooth structure
  • FIG. 2 is a partial cross-sectional view of a microwave hybrid heating system according to one embodiment of the present invention
  • FIG. 3 is a partial cross-sectional view of a MHH system according to another embodiment of the present invention
  • FIG. 4 is a flow chart showing a method for microwave sintering a green ceramic member according to one embodiment of the present invention
  • FIG. 5 is a flow chart showing a method for microwave sintering a coated ceramic member according to one embodiment of the present invention
  • FIG. 6 is an experimental time-temperature sintering profile for zirconia copings sintered in a conventional furnace at a ramp rate of 10°C per minute to 1450°C
  • FIG. 7 is an experimental time-temperature sintering profile for zirconia coping samples conventionally fast-fired at a ramp rate of 40°C per minute to 1400°C
  • FIG. 8 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Runs A-E;
  • FIG. 9 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run A;
  • FIG. 10 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run B;
  • FIG. 11 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run C;
  • FIG. 12 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run D;
  • FIG. 13 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run E;
  • FIG. 14 is an experimental time-temperature sintering profile for microwave- sintered zirconia copings from microwave sintering Run F including a dwell time of 20 minutes;
  • FIG. 15 is a chart showing the relationship between percentage density and sintering temperature for microwave-sintered samples from Runs A-E and conventionally sintered samples fired at 1450°C with a ramp rate of 10°C per minute;
  • FIG. 16 is a chart showing the relationship between Vickers Hardness Number (VHN) and the percentage density for microwave-sintered samples from microwave sintering Runs A-E and conventionally sintered samples fired at 1450°C at a ramp rate of 10°C per minute;
  • VHN Vickers Hardness Number
  • FIG. 17 is a chart showing the relationship between the indentation fracture toughness under 20Kg load testing and sintering temperature for microwave-sintered samples from microwave sintering Runs A-E and conventionally sintered samples fired at 1450°C at a ramp rate of 10°C per minute;
  • FIG. 18 is a chart showing the relationship between the indentation fracture toughness under 30 Kg load testing and sintering temperature for microwave-sintered samples from microwave sintering Runs B-E and conventionally sintered samples fired at 1450°C at a ramp rate of 10°C per minute;
  • FIG. 18 is a chart showing the relationship between the indentation fracture toughness under 30 Kg load testing and sintering temperature for microwave-sintered samples from microwave sintering Runs B-E and conventionally sintered samples fired at 1450°C at a ramp rate of 10°C per minute;
  • FIG. 18 is a chart showing the relationship between the indentation fracture toughness under 30 Kg load testing and sintering temperature for microwave
  • FIG. 19 is an experimental time-temperature sintering profile for coated zirconia copings that were microwave sintered at 800°C with a 10 minute preheat period, a ramp rate exceeding 270°C per minute between 200°C and 800°C and a one minute dwell time;
  • FIG. 20 is an experimental time-temperature sintering profile for the coated zirconia coping that was microwave sintered at 700°C with a dwell time of one minute;
  • FIG. 21 is an experimental time-temperature sintering profile for the coated zirconia copings that were microwave sintered under "rapid power" without a preheat period from room temperature to 800°C with a one minute dwell time at 800°C;
  • FIG. 22 is an experimental time-temperature sintering profile for coated zirconia copings that were microwave sintered to a temperature exceeding 800°C with no dwell time
  • FIG. 23 is an experimental time-temperature sintering profile for a plurality of microwave-sintered coated zirconia coping samples that were simultaneously microwave sintered to a temperature of about 800°C with a dwell time of one minute
  • FIG. 24 is a chart showing a comparison of the Vickers Hardness Numbers for microwave-sintered and conventionally sintered coated zirconia coping samples
  • FIG. 25 is an X-ray diffraction pattern for a sample of the opaque base shape powder
  • FIG. 26 is an X-ray diffraction pattern for a sample of the luster shade powder
  • FIG. 27 is an X-ray diffraction pattern for a microwave-sintered, coated zirconia coping sample.
  • the present invention provides a microwave hybrid heating system and methods for microwave sintering ceramic materials in a fraction of the time required to conventionally sinter the same materials.
  • the ceramic materials that are sintered in the microwave hybrid heating system according to the present invention and using the method according to the present invention have physical, aesthetic and mechanical properties that are comparable to, if not superior to, the properties achieved in conventional firing processes.
  • Fig. 2 is a partial cross-sectional view of a microwave hybrid heating (MHH) system according to one embodiment of the present invention.
  • the MHH system includes a standard microwave furnace 1 that operates at a frequency of 2.45 GHz, including a magnetron microwave source.
  • the MHH system also includes a thermal containment box 10, which is made from a material that is transparent to microwaves, even at high temperatures.
  • the material of the thermal containment box 10 preferably does not absorb or reflect microwave energy at any temperature less than or substantially equal to the maximum sintering temperature of the ceramic samples 50 to be sintered therein.
  • the particular material of the thermal containment box 10 is not critical, so long as it can withstand the sintering temperatures and contain the heat generated by the interaction of the microwaves with the materials provided in the thermal contaimnent chamber 11 thereof. Examples of suitable materials include, but are not limited to, fibrous alumina, foam silica, and boron nitride.
  • the insulative properties of the thermal containment box 10 also contribute to the overall energy efficiency of the MHH system according to the present invention, as described in more detail below.
  • the thermal containment box can have any shape, such as a cube, rectangle, or cylinder, for example, and the specific size and shape of the thermal containment box is not restricted, except by practical considerations, such as the size and shape of the microwave chamber.
  • the MHH system according to the present invention also includes a plurality of primary susceptors 20 made from a material such as SiC that readily couples to microwave energy at room temperature. Examples of suitable materials also include, but are not limited to, molybdenum disilicide (MoSi 2 ), graphite, and other high dielectric loss ferrite-type materials.
  • the primary susceptors 20 can be any shape, such as annular, disk or plate-shaped, and the particular size and shape of the primary susceptors 20 is not restricted.
  • the microwave reflecting temperature the temperature at which the material of the primary susceptors 20 tends to become reflective to microwave energy
  • the microwave reflecting temperature is less than the maximum sintering temperature of the samples
  • the secondary susceptors 30 can be made of any material that can withstand the sintering temperatures within the thermal containment chamber 11 and which couples to microwaves at a temperature (hereinafter referred to as the microwave coupling-trigger temperature) that is greater than room temperature and less than the microwave reflecting temperature of the primary susceptors 20.
  • each intermediate susceptor is preferably less than the microwave reflecting temperature for the primary susceptor and any preceding susceptors.
  • the microwave reflecting temperature of each intermediate susceptor is preferably greater than the microwave coupling-trigger temperature for the successive susceptors.
  • At least one susceptor has a microwave reflecting temperature that is greater than the microwave coupling-trigger temperature and the maximum sintering temperature of the ceramic material 50 to be sintered, so the ceramic material gains the benefit of radiant heat from a susceptor in cooperation with direct microwave sintering.
  • the MHH system shown in Fig. 2 which was also used in the experiments described below, includes a plurality of ring-shaped SiC (silicon carbide) susceptors 20 that are provided as primary susceptors, and two reticulated zirconia plates 30 provided as secondary susceptors.
  • the reticulated zirconia plates 30 are positioned on the floor 12 within the containment chamber 11 of the alumina thermal containment box 10, and the SiC susceptors 20 are sandwiched between the reticulated zirconia plates 30.
  • the thermal containment box 10 is positioned within the microwave cavity 2 of the microwave furnace 1.
  • the thermal containment box 10 includes at least one port 14 through which a thermocouple 40 is inserted to monitor the temperature within the thermal containment chamber 11.
  • the thermocouple 40 is supported by a metallic pedestal 41, however, any means for supporting and positioning the thermocouple can be used.
  • an optical pyrometer can be used in lieu of a thermocouple.
  • the silicon carbide primary susceptors 20 directly couple to the microwave energy at room temperature substantially immediately and become infrared radiant heaters.
  • the reticulated zirconia plates 30 absorb the infrared heat energy from the SiC rings 20 and further increase the temperature within the containment chamber 11 by radiating heat and acting as secondary susceptors. In that manner, the reticulated zirconia plates 30 continue to provide radiant heat to the system even after the temperature exceeds the microwave reflecting temperature of the SiC primary susceptors 20.
  • the ceramic samples 50 are initially heated by the radiant energy emitted from the SiC primary susceptors 20 and then by the radiant energy emitted from the reticulated ZrO secondary susceptors 30 until the microwave coupling-trigger temperature for the ceramic samples 50 is reached. At that point, the ceramic samples 50 directly couple to the microwave radiation for the duration of the sintering process and also gain the benefit of improved heating uniformity that is provided by the cooperative radiant heat emitted from the secondary susceptors 30. It is not necessary to provide secondary or intermediate susceptors, however, so long as the primary susceptors comprise a material which has a microwave reflective temperature that is greater than the microwave coupling-trigger temperature of the ceramic samples 50.
  • the radiant heat emitted from the susceptors cooperates with the microwave energy to provide uniform heating during sintering, even after the sample material begins to directly absorb the microwave energy itself, the external radiant heat is not required in order for the sample to realize the above-described benefits of microwave sintering.
  • the insulative properties of the thermal containment box 10 contribute to the overall energy efficiency of the MHH system according to the present invention. Unlike conventional sintering furnaces, the energy required to heat the MHH system is well contained within the thermal containment chamber 11 and not lost. In that manner, the overall energy efficiency of the system is improved over conventional systems.
  • Fig. 3 is a partial cross-sectional view of another MHH system according to the present invention. Like components have been designated with like reference numbers, which are described above with reference to Fig. 2. The embodiments shown in Figs. 2 and 3 primarily differ with respect to the configurations of the primary susceptor 20 and secondary susceptor 30.
  • a primary susceptor 20, made of SiC for example, is provided as a layer positioned on the inner peripheral surface 13 of the thermal containment chamber 11.
  • a secondary susceptor 30, such as reticulated ZrO is provided as a layer positioned on and covering the primary susceptor 20.
  • the inner peripheral sides of the thermal contaimnent chamber 11 are substantially covered by the susceptor configuration, and the primary susceptor 20 is sandwiched between the inner peripheral wall 13 of the thermal containment chamber 11 and the secondary susceptor 30.
  • this configuration is not limited to the embodiments shown in Figs. 2 and 3 wherein the thermal containment box 10 is substantially cylindrical, and modifications can be easily made to accommodate other shapes.
  • Fig. 4 is a flow chart showing a method for microwave sintering green ceramic members according to one embodiment of the present invention. At room temperature, at least one green ceramic sample is positioned on a setter, or directly on a secondary susceptor such as a reticulated zirconia plate, proximate the floor of a thermal containment chamber of a thermal containment box.
  • a secondary susceptor such as a reticulated zirconia plate
  • the thermal containment box is positioned within a microwave cavity of a microwave furnace and means for measuring the temperature within the thermal containment chamber is provided, such as a thermocouple or optical pyrometer.
  • a steep heating rate preferably greater than 50°C/minute, is employed to achieve a maximum sintering temperature.
  • the time required to complete firing is preferably less than 45 minutes, and is more preferably in a range of 29-39 minutes, depending on the maximum desired sintering temperature and the overall mass of the green ceramic members to be sintered.
  • the properties of the ceramic sintered according to this method are comparable to those achieved by conventionally sintering the samples at a slow ramp rate to a maximum temperature that is within 150°C of the maximum microwave sintering temperature.
  • a method for microwave sintering ceramic materials, such as zirconia dental copings, that have been previously sintered and then subsequently coated with one or more layers of a ceramic enamel or a glass-ceramic type composition.
  • the method includes the steps of placing the coated sintered ceramic sample in the thermal containment chamber of the thermal containment box of the MHH system at room temperature in the same manner described above with respect to Fig. 3.
  • An initial preheat period is provided, the duration of which varies depending upon the mass of the samples to be sintered and during which time the temperature within the thermal containment chamber is gradually raised by the radiant heat emitted from the primary susceptors.
  • the thermal containment chamber is then heated at a steep rate, preferably exceeding 150°C/minute, to the desired maximum sintering temperature of the coated ceramic sample, which is preferably about 800°C for the specific examples described herein.
  • a dwell period of about 1 minute is preferably provided.
  • the time required to reach the maximum sintering temperature according to the method shown in Fig. 4 is preferably less than 30 minutes, and more preferably, in a range of about 14-24 minutes, depending upon the mass of the samples and the pre-heat period provided, with a major portion of this time being provided for the pre-heat period.
  • the maximum sintering temperature of 1450°C was reached in approximately two and half hours.
  • the cooling ramp rate was also about 10°C per minute to approximately 800°C.
  • the cooling rate then slowed gradually over the next two hours to ensure that samples were not subjected to large thermal stresses.
  • Conventional sintering of ceramic copings in dental labs takes approximately the same amount of time, depending on the actual heating and rates used. As shown in Fig. 6, it took approximately 1000 minutes, or roughly 16.6 hours, to complete the firing and cooling cycle from room temperature to room temperature.
  • the samples that were conventionally sintered at a heating rate of 10°C per minute were densely sintered, however, and exhibited a translucent sheen that indicated that they were fully dense.
  • Microwave hybrid heating was employed to sinter samples.
  • a platinum sheathed S-type thermocouple and an Omega controller were procured from Microwave Research and Applications, INC (Model number BP 210/211).
  • a thermal fibrous alumina cylinder containment box was made from fibrous alumina boards one and a half inches thick, and 3 silicon carbide rings (25 mm in diameter) were used as primary susceptors.
  • the containment box included a lid that could be opened and through which samples were introduced into the containment box.
  • SiC primary susceptors were sandwiched between reticulated zirconia plates.
  • the reticulated zirconia plates were used a secondary susceptors and also as setters in that these plates were provided upon the floor of the thermal containment chamber to provide the floor upon which the samples rested in the microwave.
  • Two stainless steel pedestals with equally spaced 4 mm holes drilled in a direction perpendicular to the vertical axis were used to accommodate the S-type thermocouple.
  • An 8 mm hole was drilled horizontally into the side of the cylindrical portion of the thermal containment cylinder.
  • a fitted pyrolytic boron nitride sleeve was inserted to line the hole of the cylinder to ensure that no arcing would occur between the box and the thermocouple.
  • thermocouple probe was inserted to clear the aligned hole and allow temperature measurement within the cavity of the thermal containment box.
  • the placement of the thermocouple for accurate temperature measurements is also critical. That is, due to the high sintering temperatures, the possibility of arcing between the thermocouple and the sleeve exists, which would result in improper temperature measurements.
  • the thermocouple tip (about 0.5 inches) was positioned right inside the thermal containment box to accurately measure the temperature.
  • the actual " sample temperature might be 50-100°C higher than the temperature displayed by the the ⁇ nocouple read out for several reasons. That is, the thermocouple measured the heat contained within the thermal containment box and not the actual heat to which the samples are subjected.
  • thermocouple used was sheathed in platinum, which acts as a thermal barrier between the sample and the thermocouple and a thermal bridge between the sample and the exterior.
  • Table I The experimental microwave sintering temperatures for microwave sintering Runs A-G are shown in Table I. Temperatures were manually recorded every two minutes. The microwave furnace was started at a minimal power level and the power level was gradually increased, as dictated by the desired ramp temperatures, until the maximum sintering temperature was achieved. The microwave furnace used did not have power feedback control.
  • the microwaves were initially applied, they passed through and out of the thermal containment box, because the thermal containment box was made of fibrous alumina, which is a ceramic material that is transparent to the microwaves due a low thermal conductivity, but energy was retained within the box.
  • the silicon carbide susceptors coupled well with the microwaves due to their high susceptibility to microwaves at room temperature and transformed this energy to infrared energy, which was emitted as heat.
  • the temperature of the system increased. The heating rate was sufficiently high to realize high temperatures such that effective coupling of microwave energy with zirconia (both the reticulated zirconia plates and the sample) was achieved at the critical couple-triggering temperature.
  • the zirconia became more susceptible to microwave energy, thereby absorbing the microwave energy at a greater rate than that of the silicon carbide susceptors. That is, once the temperature of the system was such that the SiC susceptors became reflective to the microwaves and discontinued radiant heat emission, the heat emitted from the secondary susceptors maintained the temperature of the thermal containment chamber and provided external radiant heat to the zirconia samples which, by that point, were also being internally and directly sintered by the microwave energy itself. Thus, in this microwave hybrid heating system, the silicon carbide primary susceptors immediately coupled to the microwave energy and became infrared radiant heaters.
  • the reticulated zirconia plates absorbed this thermal energy and increased the temperature of the system by radiating heat and acting as secondary susceptors to provide radiant heat after the microwave coupling-trigger temperature for the reticulated zirconia was met.
  • the stabilized zirconia samples were heated by the radiant energy, initially emitted from the primary susceptors and then by secondary susceptors, until the couple- triggering temperature was met, whereby the stabilized zirconia sample material directly coupled to the microwave energy for sintering in cooperation with the radiant heat from the secondary reticulated zirconia susceptors.
  • the ' dielectric loss factor of the silicon carbide increased rapidly and the SiC susceptors became reflective to microwaves.
  • the microwave couple-triggering of zirconia is in a range of 300-600°C, depending upon the particular compositional ingredients present in the material, such as stabilizers or impurity elements.
  • the stabilized zirconia samples and reticulated zirconia plates were heated to their respective microwave coupling-trigger temperatures by the infrared radiation emitted from the silicon carbide primary susceptors, after which point the zirconia plates merely provided additional radiant heat for the system and the zirconia samples were heated by the thermal energy radiated from the secondary susceptors as well as direct microwave radiation until they were sintered. It should be noted that the use of the steel pedestals to support the thermocouple within the microwave chambers in the experimental MHH set-up did not affect the sintering temperatures.
  • Run A (Microwave Sintering at 1100°C): Four samples were sintered at 1100°C Fig. 9 shows the time-temperature- power profile for Run A, and shows that the ramp rate was in excess of 100°C per minute. A maximum power of was 72 % the maximum potential output was required to achieve the maximum sintering temperature of 1100°C in a time of twenty-six minutes. The cooling rates were also constant and in excess of 100°C per minute. The entire sintering cycle was completed in less than 35 minutes. However, upon removal of the samples after cooling, they were not completely sintered, and had warped to a certain extent.
  • Run B (Microwave Sintering at 1300°C): Fig. 10 shows the sintering profile used to sinter the Run B samples to a maximum temperature of 1300°C. The maximum power required was 78 %. During this cycle, however, the temperature rose very slowly from room temperature to about 150°C at a heating rate of 10°C per minute. Increasing the power to 73 % raised the ramp rate significantly until a temperature of about 300°C was reached, and then the delivery of 78 % power further increased the ramp rate to more than 150°C per minute.
  • the Run B samples were dense and did not show any warping. The normally expected cracks, i.e., those which ordinarily occur when samples are subjected to thermal shock, did not occur here.
  • Run C Microwave Sintering at 1350°C: As shown in Fig. 11, when the temperature started to waiver around 200°C, the power was increased to 70 %. The temperature then increased from 150°C to 1000°C at a rate of 71°C per minute. This increased ramp rate is lower than the ramp rates of Runs A and B, which exceeded 100°C per minute. Upon examination, the cooled samples appeared to be completely sintered.
  • Run D (Microwave Sintering at 1400°C): The power was manipulated until the temperature reached about 380°C. As shown in Fig. 12, the final power level of 72 % increased the microwave heating such that a heating rate of approximately 150°C per minute was employed until the maximum sintering temperature of 1400°C was reached. Upon examination, the cooled Run D samples were intact, did not show any signs of cracking and had a desirable translucent sheen, which was not observed in the samples from the sintering profiles followed in Runs A-C. While the possibility of thermal stress generation is always a reality with such rapid heating rates, the heating and the cooling rates were almost equal. Moreover, for the first nineteen minutes of the sintering run, the heat of the thermal containment box provided the initial sintering mechanisms for the samples, such that when the high heating rate was applied, the samples readily absorbed the microwave radiation and sintered rapidly.
  • Run E (Microwave Sintering at 1450°C): Fig. 13 shows that the Run E time-temperature profile represented a more uniform heating curve with a ramp rate of approximately 60°C per minute compared to the microwave sintering profiles of Runs A-D. A maximum power of 72 % was adequate to reach the sintering temperature of 1450°C. The cooling rate followed the same schedule as the previous cycles. The overall time required to complete the cycle was extended, however, due to the slower ramp rate. The ten samples sintered according to this cycle displayed a desirable translucent sheen and were completely sintered.
  • Run F Microwave Sintering at 1200°C with Dwell Times of 20 and 30 Minutes:
  • One potential benefit of using microwave sintering systems is that such a system may enable lower temperature sintering. As shown in Run A, however, a low temperature of 1100°C, with no dwell time was insufficient to sinter the samples. Thus, Run F examined the results of microwave sintering at a relatively low sintering temperature of 1200°C with a dwell time. Fig. 14 shows the Run F sintering profile including a dwell time of 20 minutes. The power was increased as necessary to cause a corresponding temperature increase. Since the microwave furnace that was used lacked water-cooling mechanisms, it was difficult to hold the temperature at 1200°C for a prolonged period.
  • the Teflon floor of the microwave oven expanded, forming a bubble below the thermal containment box.
  • This phenomenon was attributed to the fact that when microwaves pass through and out of the thermal containment box, heat is contained within the box because of low thermal conductivity of the fibrous alumina.
  • the outside of the containment box was cooler than the inside. For example, the temperature at the outside region may be around 200°C, compared to the inside of the box (1350°C). Further, there was not complete air circulation at the bottom of the box to compensate for the heat that dissipated below the box.
  • the coefficient of thermal expansion for Teflon (i.e., the floor of the microwave) is very high (12-13 inch/inch/°C). Because of the high coefficient of thermal expansion of Teflon, for every degree of temperature increase there is corresponding increasing in length until it expands and buckles. Thus, when heat flows from the box, the Teflon expands either above or below the box; in this case it did so by forming a bubble. For the run including a dwell time of 30 minutes, the containment box was repositioned away from the buckled areas of the microwave floor, but the results were similar. Further runs with dwell times were not performed.
  • Run G Microwave Sintering at 1700°C: The initial power was set at 75 %. After two minutes, the power was increased to its maximum of 100 %. The heating ramp rate exceeded 150°C per minute, and the sintering temperature of 1700°C was reached in about ten minutes. Although these microwave-sintered samples were completely dense, this high sintering temperature damaged the microwave furnace such that the floor was completely burned.
  • the conventional and microwave sintered cylinders were measured using a micrometer with respect to their length, diameter, and the samples were also weighed. The differences between the pre-sintered and the post-sintered samples were recorded. There was not any significant, statistically measurable difference between the post- sintering dimensions (i.e., shrinkage characteristics) for conventionally sintered and microwave-sintered samples.
  • Indentation hardness numbers and fracture toughness are extensively used in evaluating the potential perfonnance of dental materials.
  • a Vickers Indentation hardness tester was used for this study.
  • the Indentation fracture technique has been established as an accurate procedure for determining the approximate fracture toughness for brittle materials.
  • Three samples from each lot for each of the microwave sintering Runs A-E shown in Table I were measured, and six of the conventionally fired samples that had the highest percentage density were measured.
  • Sample Preparation for Hardness and Toughness Measurements The samples were each sectioned by a diamond blade, and mounted on a thermosetting epoxy mount. The samples were then polished with grit progressing from 120, 240, 340 and 600. The final polishing was performed with a series of decreasing diamond pastes from 30, 15, 6 and l ⁇ m. Times were varied from one to five minutes at each stage. The samples were observed under the optical microscope to check for any polishing scratches and, if present, the samples were re-polished to ensure accurate measurements.
  • Static Hardness and Indentation Toughness Measurements Static hardness testing was done using a Vickers Hardness Tester. Indentation loads of 20 Kg and 30 Kg were employed with a hold time of 5 seconds for each sample during indentation. Five indentations were made on each sample. The mean diagonal length and the corresponding crack length were measured using the guage meter attached to the equipment. The Vickers Hardness number and the indentation fracture toughness were calculated as follows:
  • H v 1.854 P/ d 2 ; Where 1.854 is the geometric factor for Vickers indenter (from sin 68°), P is the test load (in kg) and d is the mean of both diagonals (in mm).
  • the sintering temperature influenced the indentation hardness and the fracture toughness values for the various microwave-sintered samples under both 20 Kg and 30 Kg loads. As shown in Fig. 16, a linear relationship exists between the Vickers hardness number and the percentage density of the sintered samples tested under 20 Kg loads. Table III shows the relationship between the Vickers hardness values for both 20 Kg and 30 Kg loads and the corresponding sintering temperatures for the microwave sintering runs. The hardness numbers were significantly lower for samples that were exposed to microwaves for short periods of time. The hardness values for samples microwave- sintered at 1450°C (Run E) are significantly higher than that of conventionally sintered samples at 1450°C.
  • the hardness values for microwave-sintered samples of Run D (1400°C) were also comparable with that of the conventionally sintered samples at 1450°C.
  • the hardness values of the microwave samples sintered at 1450°C are comparable with that of the published values for conventionally sintered zirconia when used as a dental biomaterial.
  • the volumetric heating by the microwaves resulted in better mechanical properties when compared to the conventionally sintered samples.
  • increasing the ramp rate as in case of the microwave-sintered samples, did not have any deleterious effect when compared to the slow ramp rate of conventional sintering. Since natural teeth are frequently subjected to stresses from 50 N to 500 N, any dental material that replaces natural dentition should also be able to withstand such forces.
  • Fracture Toughness Results The fracture toughness was calculated from the measured radial cracks around each indent made under HV 2 o and HV 30 loads. The fracture toughness increased with sintering temperature as shown in Fig. 17.
  • the HV 20 indentations of microwave-sintered samples from Run E were comparable with those of the samples that were conventionally sintered at 1450°C.
  • the increase in the standard deviation for conventionally sintered samples is attributed to non- uniform grain size in these samples. Cracks that were measured at the center of the samples propagated more than those measured at the edges of the samples.
  • the transformation toughening mechanisms imparted to zirconia that is stabilized with 3 mol % yttria increases the resistance to crack propagation and results in higher fracture toughness.
  • the amount of transformed grains, or the size of transformation zone depends upon the transformability of the tetragonal phase, which is controlled mainly by the grain size. While microwave sintering resulted in uniform and smaller grains, more grains transformed and absorbed the energy of the crack, which resulted in slightly higher fracture toughness values. Also, the fracture toughness values are influenced by the mode of crack propagation.
  • the mechanical properties of the samples are influenced by the microstructure, but are also influenced by defects, as well. Both the details of the microstructure and the presence of defects were microscopically examined.
  • the samples were removed from the epoxy mounts and cleaned in an ultrasonic bath of deionized water for 30 minutes to remove residual epoxy material. The samples were either thermally or chemically etched for examination under the microscope. Microwave Sintered samples from Runs A (1100°C), B (1300°C) and C (1400°C) were etched with 49 % Hydrofluoric acid for 3 minutes.
  • the conventionally sintered and the other microwave-sintered samples were thermally etched to a temperature of 100°C less than the sintering temperature and held for one to one and half hours (more for the microwave-sintered samples).
  • the samples were sputter coated for 3 minutes to provide a 300 A thick coating of gold palladium and then examined under a Scanning Electron Microscope.
  • the conventionally sintered samples were examined with an ETEC while the microwave- sintered samples were examined using a PHILIPS electron microscope.
  • microstructure of the dental ceramic samples consisted of granular aggregates that are typically seen for these types of ceramics.
  • the average microstructure of the microwave-sintered samples had less voids than the conventionally sintered samples at the same magnification of 10000 X. No significant difference in grain shape was observed between the two sintering methods.
  • the samples that were conventionally sintered samples at a ramp rate of 10°C per minute had slightly larger grains (approximately 0.7 ⁇ m) in the interior than the exterior, approximately 0.3 ⁇ m. This indicated that the temperature of the external surface might have been higher than the interior. Few voids were also seen in these samples.
  • the microwave-sintered samples had uniform grain size distribution, which was closely packed, but had less porosity than the conventionally sintered samples.
  • the microstructure was more uniform throughout the sample and confirms the volumetric heating phenomenon.
  • the average grain size was approximately 0.3 ⁇ m.
  • Thermal etching brought out excellent microstructural features.
  • Chemically etching the microwave-sintered samples did not enhance the reproducibility of the granular microstructure. Most of the grains appear to be chewed up.
  • the microwave- sintered sample from Run A (1100°C) revealed the presence of fine cracks, which decreased the fracture toughness values. This was mainly due to improper sample preparation rather than thermal stresses.
  • the microstructure of the microwave-sintered samples from Run D (1400°C) included a few slightly larger grains (0.5 ⁇ m) interspersed among the densely packed smaller grains ( ⁇ 0.3 ⁇ m).
  • the microstructure included uniform crystal distribution in the glassy matrix for the teeth sintered gradually as compared to the rapid sintering by microwave. However, the degree of crystallinity of the gradually fired microwave sample is higher than those of rapidly fired samples. Thus, by varying the dwell time, the size and amount of crystalline phase can easily be controlled during the sintering process. Indentation hardness and fracture toughness measurements were conducted under HV 0.5 and HV 0 . 2 loads.
  • Sample Materials Six anterior (front teeth) copings and ten posterior (molars or chewing teeth) copings, made from sintered zirconia stabilized with 3 mol % yttria, were provided by Town and Country Dental Laboratory (Freeport, NY) and Nobel Biocare (Yorba Linda, CA). Town and Country Dental laboratory also provided the dental glass-ceramic (NORITAKE, Japan) coating materials that were compatible with the zirconia copings.
  • the dental glass-ceramic materials for enameling the copings were supplied in the form of fine powders, which were mixed with modeling liquid and condensed into the desired form. The particle size distribution of the powders play a key role in producing the most dense packing when the powders are properly condensed.
  • the zirconia copings were cleaned with de-ionized water in an ultrasonic bath. Four basic shades of dental glass-ceramics were used in this experiment: the shade base; a dentine shade; an enamel shade; and a luster shade.
  • the different colors of the layers are provided by pigmented oxide additives, i.e., the blue appearance of the enamel shade was due to the addition of cobalt oxide and the opacity of the opaque base was due to the presence of zirconium, titanium, or tin oxides.
  • the forming liquid, or modelling liquid (starch and sugar along with distilled water), was mixed with each shade as described below. Excess water was blotted away from the layers to ensure optimum powder packing on the zirconia copings.
  • a soft grip (SG-250) was used to mix the cement and the forming liquid on a dental cement mixing pad.
  • the shade base that is, the opaque layer
  • the consistency of the mix was important and care was taken to obtain the optimal consistency and texture of the mix.
  • a mixing ratio of one part powder to one part forming liquid was used.
  • Application of the layers of the dental glass- ceramic in their right consistency was also crucial to obtaining dense particle packing of the powder.
  • the thick creamy pasty mix was not allowed to drop off of the applying medium.
  • the copings were held with tweezers, and the opaque shade was applied to the copings by the mixing brush. The tweezer was lightly tapped during the application of the opaque shade.
  • the soft grip brush coated the layers very evenly, however, a spatula can be used to smooth the wet dental glass-ceramic.
  • a spatula can be used to smooth the wet dental glass-ceramic.
  • the dentine shade was mixed and applied in the same manner as described and applied over the first layer. Similarily, the other two layers were mixed and applied accordingly to the manufacturer's instructions and the final carving of the occlusal anatomy (tooth countour) was made by a sharp probe. Overcoating was avoided in order to reduce the occurrences of uneven surfaces.
  • the coping layered with the dental glass- ceramic was placed into the furnace (automatically, given furnace automation) and was heated to 900°C at rate of 90°C per minute. After the firing cycle was completed, the restorations were removed and examined. The second layer of dentine and enamel shades was applied and the same firing cycle was repeated. Upon cooling, the restoration was examined and, again, due to lack of consistency, individual shades were reapplied and a slower heating rate of 70°C per minute was employed. Only the enamel and the luster shades were applied and re-fired with a rate of 50°C per minute until the optimum smooth finish was obtained.
  • the other coping layered with the dental glass-ceramic was subjected to a single firing with faster heating rates in the same manner as the microwave sintering.
  • the heating rate was 100°C per minute.
  • the conventional firing cycle mentioned above was initiated and the sample was sintered to 925 °C and allowed to complete its cycle. The shades of enamel and luster had to be reapplied two additional times and firing was conducted at a much slower heating rate of 50°C per minute.
  • Pre-heating is required because placement of the condensed coating materials directly into a warm furnace would result in rapid production of steam, thereby introducing many voids that would ultimately fracture the sections of the tooth.
  • the firing cycle was initiated.
  • the faster ramp rate of 90°C per minute enabled a temperature increase from 500°C to 925°C within a period of five minutes.
  • the cooling rates were also about 90°C per minute.
  • the restoration was removed.
  • the restoration appeared cracked and was highly porous.
  • the major content of the glass-ceramic powder was leucite, and changes in the leucite content caused the generation of sufficient stresses that led to cracking in the porcelain.
  • the bond strength between the layers appeared to be very weak and they chipped easily. Since the layers could easily be removed, the enamel and the dentine shades were recoated on the coping.
  • the sample with the reapplied layers was sintered along with the other sample using the standard procedure for firing dental crowns.
  • Incremental firing pertains to the individual application and sintering of each layer of the dental glass-ceramic.
  • the opaque base was applied, and the sample was dried in front of the open door of a pre-heated furnace. When completely dry, the sample was automatically inserted into the furnace and then fired with a ramp rate of 50°C per minute to 925°C.
  • the sample was automatically cooled by the furnace operational system and the tray was removed with a long pair of tongs. This process took around twenty-five minutes. During cooling, a few bare spots were noticed, which had to be re-covered with sufficient opaque shade, since smooth surfaces are required to produce strong bonds with the other layers.
  • the sample was placed into the furnace following the same procedure.
  • Microwave Sintering of the Enameled Copings Each sample was placed upon the reticulated zirconia in the thermal containment box and the same Microwave Hybrid Heating (MHH) technique described above was used to microwave sinter the coated samples. Temperature measurements were taken every two minutes. The power was increased gradually for few samples and for few other samples the power was increased rapidly (rapid bursts) as shown in Table V. The sintering temperature was about 800°C with a hold time of one minute. One sample, however, was fired to approximately 700°C with a hold time of one minute. Table V. Experimental Thermal Schedules for the Coated Copings
  • the posterior coping samples were enameled with the layers of the dental glass-ceramic. These samples were fired with the power-temperature profile that worked well with the anterior tooth copings. One at a time, four teeth were sintered with the preferred power-profile shown in Table VI. The remaining six coping samples were placed on the reticulated zirconia floor in the thermal containment box and gradually fired to approximately 800°C according to the thermal schedule shown in Table VIII.
  • Microwave Sintering Profile for the Enameled Copings As described above, the conventional method of firing the layered (enameled) dental copings involves multiple sintering runs with sintering temperatures on the order of 900-960°C and dwell times of about three minutes for each of the incremental runs.
  • the present invention processing technology facilitates the use of lower temperatures and a reduced sintering period, and allows many crowns or restorations to be fired at once.
  • the microwave sintering temperatures ranged from 700-800°C, with a dwell time of one minute.
  • the overall experimental sample profile is shown in Table V.
  • an initial soak period at a low temperature was provided to ensure that the water in the layered dental ceramic materials was removed. This procedure is comparable to conventional methods, where the copings layered with dental ceramics are placed in the muffle of a pre-heated furnace for few minutes to remove water prior to sintering.
  • Microwave Sintering at 800°C The temperature-power-time profile of Fig. 19 shows that the initial ramp rate was around 40°C per minute.
  • the microwave-sintered samples were exposed to a pre- heat period, during which the temperature varied for ten minutes. At 200°C, the power was increased and a ramp rate in excess of 270°C per minute brought the sample to the sintering temperature. The temperature was maintained at about 800°C for a one-minute dwell period, and then the microwave power was terminated.
  • the cooling rates were not controlled, but were approximately 110°C per minute. After the furnace cooled to a temperature below 150°C, the furnace was opened for further cooling. The entire sintering process was completed less than 25 minutes.
  • Cooling rates in excess of 100°C per minute had no deleterious effect (e.g., cracking) on the sintered restorations. This is a major advantage of using microwave sintering in making dental restorations. It should be noted that the thermochemical reactions between the ingredients occurred during the original manufacturing process for each of the powdered ingredients, and that the purpose of the firing was merely to ensure the proper fusion of the powders to completely form the restoration. The restorations exhibited an excellent finish with natural translucent surface.
  • Microwave Sintering at 700°C The primary objective of sintering samples at a temperature of 700°C was to determine if microwave process would enable restorations to be sintered at temperatures that are about 250°C less than the final sintering temperatures for conventional firings (920-950°C).
  • the thermal schedule represented in Table VI shows that the schedule included a pre-heating period of fifteen minutes to evaporate the water in the layered restoration. Raising the microwave power level to 68% was sufficient to achieve a maximum sintering temperature of 700°C with a dwell time of one minute, as shown in Fig. 20. The microwave switched off after the dwell period. The cooling rates were similar to prior schedules. The restorations were removed when the temperature was below 150°C. Table VII. Thermal Schedule for Enameled Coping Microwave Sintered at 700°C
  • the finished restoration did not have a good finish and had porous appearance compared to the sample fired with the 800°C firing schedule shown in Table VI.
  • Microwave Sintering with Rapid Power Increase In this run, the maximum power was set to 75%, and no preheat period was provided before the sample was sintered at 800°C. The sintering temperature was achieved within five minutes using a heating ramp rate of more than 275 °C per minute. The cooling rate was of 100°C per minute, as shown in Fig. 21. The samples exhibited improper sintering with very rough, cloudy surfaces. This result is attributed to temperature being raised too quickly, that is, before the ceramic layers vitrified, such that complete sintering could not be accomplished. Microwave Sintering with No Dwell Period: Dwell periods of two to three minutes in conventional sintering are required to allow complete thermochemical reactions to take place.
  • thermochemical reactions without any dwell periods.
  • the ramp rate was increased to 100°C per minute until the sintering temperature was achieved, as shown in Fig. 22.
  • the cooling rates were similar to the heating rates.
  • the sample exhibited complete sintering in patches. Areas near the margins of the sample were porous (cloudy), but the occlusal surface of the sample appeared to have a good translucency. Based on this, dwell periods are preferred to provide complete vitrification.
  • the remaining two samples were sintered with the preferred sintering profile as described by Table VI, with a dwell of one minute. The overall aesthetic results were excellent.
  • Microwave Sintering of Multiple Crowns The copings obtained from Nobel Biocare Inc. (Yorba Linda, California) were used in multiple crown microwave sintering experiments. Eight copings were enameled with layers of dental glass-ceramic as described above. The crowns were positioned on the reticulated zirconia floor of the thermal containment box in a horseshoe arrangement surrounding the thermocouple. Due to the large number of samples, the preheat period was raised to around twenty minutes as shown in the thermal schedule in Table VIII. The power was increased to 75% after the initial preheat period was completed because the larger sample load required more power. The maximum sintering temperature was around 810°C, and over a period of twenty seconds, the temperature dropped to 798°C.
  • the power for the microwave furnace was switched off after the one-minute dwell period.
  • the sintering process was completed in a period of thirty minute as shown in Fig. 23.
  • Each of the restoration samples was completely sintered and had excellent translucency. This confirmed the practical applicability of microwave sintering to produce sinter many crowns at once with a gradual thermal profile.
  • the number of crystals, the size and the growth rate are regulated by the dwell periods of the sintering temperature. This explains the dwell periods of two to three minutes (sometimes even five) conventionally followed by the dental technicians.
  • Examination of the microstructure of the microwave-sintered sample fired with the preferred thermal profile shown in Table V showed that leucite crystals (KalSi O 6 ), wollastonite (CaSiO 3 ) and calcium aluminum silicate (CaAlSiO ) were precipitated in the glassy matrix of the material (some elongated and some circular). These crystals were determined by X-ray Diffraction. Single crystal formation is typical of this type of microstructure. The formation of such a microstructure can be attributed to the actual sintering process.
  • the glassy matrix did not have sufficient time to dissolve completely and resulted in an irregular matrix characterized by pits and craters. A few crystals can be seen protruding from the glassy matrix. Also, at higher magnification, few areas of cloudy formations were seen, which are considered to be the result of incomplete precipitation of crystals that were entrapped within the glassy matrix due to rapid sintering which did not allow sufficient time for complete precipitation.
  • the results of the microstructural examination confirmed the fact that microwave sintering results in the same mechanism of crystal formation from the glassy matrix. Needle-like appatite crystals exhibit good chemical durability, and are therefore ideal for dental materials.
  • Indentation Hardness The most important property to compare processing technologies for restorative materials is the property of hardness. Hardness is the measure of resistance to permanent surface indentation or penetration. The importance of hardness measurements in restorative dentistry is that it signifies the abrasiveness of a material to which the natural dentition will be subjected. Thus, the mechanical properties of the sintered teeth samples were evaluated using indentation tests involving both a spherical indenter (dynamic testing) and diamond shaped indenter (static indention).
  • Brinell testing has been used extensively for determining the hardness of metals and metallic materials used in dentistry. In view of the significant elastic strain exhibited by brittle materials, however, Brinell testing is not recommended to determine the hardness of dental crowns.
  • the American Dental Association recommends only Vickers hardness testing for dental crowns, dental cements or dental plastics, or those materials, which exhibit elastic recovery. Only two samples were subjected to Brinell hardness testing, since this testing method is very technique sensitive and typically requires completely flat samples.
  • a Brinell Dynamic hardness tester fitted with a 2 mm spherical indenter was used. The computer controls on the system recorded the data. A 200g load was applied to indent the samples.
  • a Vickers micro-hardness indenter was used to measure the Vickers Indentation number and Indentation Fracture Toughness. Five samples were indented using a diamond indenter with maximum load of 200g, and the indent was maintained for a period often seconds. Three samples were indented under a 500g load that was maintained for a period often seconds. The indented surfaces were verified on a television monitor attached to the equipment. When a good indent was made, an image was produced using ImagePro software. The mean diagonal length and the crack length were measured using the software. The Vickers hardness number and the Indentation fracture toughness were calculated according to the formulae described above.
  • the indentation fracture toughness is an invaluable tool for studying the behavior and properties of brittle material because the crack growth parameter is identical to those cracks expected in clinical conditions.
  • the loads of 200g and 500g introduced flaws of controlled size, shape and location that enabled direct measurements of the flaws. As a result of using the ImagePro software program to measure the flaw sizes, the obtained values are accurate.
  • the indentation hardness and fracture toughness values obtained compare well with literature reported values. Significant differences between the hardness values obtained at low loads, and the hardness values obtained at high load were observed for the samples.
  • the indentation fracture toughness was measured from the radial cracks propagating from the corners of the indents.
  • Fig. 24 shows a comparison between the VHN values obtained for the conventionally fired samples and the microwave-sintered samples that were rapid fired and fired according to the preferred profile.
  • the average fracture toughness value for the microwave-sintered samples that were fired according to the preferred profile was 2.26 ⁇ 0.08 MPa(m) 0,5 and compared well with the conventionally sintered sample, which was 2.25 MPa(m) 0,5 .
  • the rapidly fired microwave sample displayed a lower fracture toughness of 0.6 MPa(m) 0,5 , which was expected considering the porous sample surface. It is also noteworthy that cracks were generated solely by the application of load, because no original surface cracks were observed. Some indents made at 200 g were often too light and the radial cracks were not well defined. The edges of the indents for the porcelain were not as sharp as those seen on the zirconia cylinders. This may be due to the high indent loads used and the sample preparation that resulted in well-defined indents. Yet, the small crack sizes correlate well with the fracture toughness property because it controls the materials response during processing, grinding, erosion and wear.
  • X-Ray Diffraction The dental glass-ceramic powders were analyzed using X-ray diffraction (XRD) to determine the crystalline phases present in the sample and to obtain the composition of the powders.
  • XRD data was obtained with Cu-K ⁇ radiation using a Siemens D500, a 40 kV accelerating potential, and a 30 mA current.
  • a 2-theta angle scanning range was set for 10°-70°, with a step size of 0.04° and a dwell time of 10 seconds.
  • the resultant spectral peaks were analyzed using the JADE computer program.
  • XRD data of the microwave-sintered samples was obtained using a Siemens (Kristalloflex 810) low angle reflectometer.
  • a 2-theta angle scanning range was set for 10°-150°.
  • Fig. 25 shows the XRD pattern obtained for the opaque base glass-ceramic sample.
  • the XRD patterns matched well with the PDF #06-0266 and #38-1423 indicating that the glass-ceramic is probably analogous to a Leucite and Zircon tetragonal phase.
  • the leucite is the main crystalline phase that also imparts strength and precipitates as needle-like crystals from the glassy matrix. It also lends chemical stability to the restoration and prevents the restorations from attacks by oral fluids, which can dissolve restorations.
  • the addition of the pigmenting oxide, zircon is added to achieve opacity.
  • Fig. 26 depicts the XRD of the luster shade of the glass-ceramic.
  • Lithium silicate was identified as the main phase present in the luster shade that corresponded with the PDF #49-1049.
  • XRD analysis of the sintered coated coping samples identified the presence of a leucite phase, wollastonite and calcium aluminum silicate (CaAlSiO ). The trace amount of various phases was difficult to model due to low levels and broad peak shapes.
  • Fig. 27 shows the XRD pattern.
  • Flexural Strength Testing Important test parameters for determining the strength of brittle dental materials by flexural testing are the specimen thickness, the contact zone at loading, the homogeneity and porosity of the material, and the loading rate. ISO standards (ISO 6872, 1984) are used by many researchers and require a three-point or a four-point bending test for the evaluation of the modulus of rupture (MOR) of dental porcelains. Dental crowns and prostheses are more complicated than beams, but the principles are same. Laminated all-ceramic prostheses are weak when compared to simple feldspathic prostheses when low strength feldspathic surfaces are subjected to tensile forces.
  • layers of the dental ceramics were layered or enameled onto the zirconia substrates using the same methods described above for the enameled coping samples.
  • the total thickness of the coating was 0.5-0.8 mm.
  • Twenty substrates were coated; ten designated for conventional sintering and ten designated for microwave sintering.
  • the preferred firing schedule shown in Table V used for sintering the enameled coping samples was employed in microwave sintering these substrates. The power was increased as dictated by the temperature requirements. The remaining ten samples were sintered in the conventional furnace. The specimens were re-fired to achieve a natural glaze.
  • the layered beams were formed with layers of veneering porcelain and core materials.
  • the reference ceramic materials used were ICZ (Inceram Zirconia), IC (Inceram Alumina), and ICS (Inceram Magnesium Aluminum Spinel).
  • MOR modulus of rupture
  • the drift in the values may be attributed to the weak interfacial bond strength between the layers of the glass-ceramic and the substrate that delaminated and caused early failure.
  • the exact processing history of the zirconia substrates was not known.
  • the specimen thickness was very detrimental for the flexure strength values as depicted in Table XI , and the MOR was inversely proportional to the square of the specimen thickness, which resulted in higher values for the core. Nevertheless, the mean flexure strength values of the microwave-sintered materials are comparable with the flexure results of other researchers.

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Abstract

L'invention concerne un procédé de frittage de matériaux céramiques faisant intervenir un système de chauffage hybride micro-onde. Ce procédé consiste à: fournir un élément céramique à fritter; fournir un four micro-onde; fournir un bloc de conteneur thermique qui comprend un matériau virtuellement transparent à l'énergie micro-onde; fournir au moins un matériau interactif comprenant un matériau qui fusionne directement avec l'énergie micro-onde à température ambiante sensiblement aussitôt dans le bloc de conteneur thermique; placer l'élément céramique dans une chambre de conteneur thermique proche du matériau interactif; et enfin, exposer le bloc de conteneur thermique au rayonnement de l'énergie micro-onde. Le matériau interactif fusionne avec l'énergie micro-onde, ce qui a pour effet de produire de la chaleur dans le bloc de conteneur thermique. La température de l'élément céramique augmente alors pour atteindre la température de déclenchement de fusion micro-onde. A ce moment précis, l'élément céramique est fritté, sous l'effet de l'énergie rayonnante émise par le matériau interactif et de l'énergie micro-onde avec laquelle il fusionne directement.
EP04782755A 2003-09-10 2004-09-01 Procede de traitement micro-onde de ceramiques et systeme de chauffage hybride micro-onde associe Withdrawn EP1665889A4 (fr)

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WO2006103697A1 (fr) * 2005-03-31 2006-10-05 Bharat Heavy Electricals Limited Traitement thermique rapide et homogene d’un echantillon metallique important utilisant des micro-ondes haute puissance
GB0506833D0 (en) 2005-04-05 2005-05-11 Rolls Royce Plc Ceramic core bodies
EP2182881A4 (fr) * 2007-08-09 2012-04-04 Byung Kwan Kim Four de frittage à micro-ondes et méthode de frittage de dents artificielles l'utilisant
EP2143697A1 (fr) * 2008-07-11 2010-01-13 Ivoclar Vivadent AG Processus de densification de céramiques et appareil correspondant
DE102008035240B4 (de) * 2008-07-29 2017-07-06 Ivoclar Vivadent Ag Vorrichtung zur Erwärmung von Formteilen, insbesondere dentalkeramischen Formteilen
DE102008035235B4 (de) * 2008-07-29 2014-05-22 Ivoclar Vivadent Ag Vorrichtung zur Erwärmung von Formteilen, insbesondere dentalkeramischen Formteilen
EP2395307B1 (fr) * 2009-02-06 2016-04-13 Panasonic Intellectual Property Management Co., Ltd. Procédé de cuisson par microondes, et four de cuisson par microondes
CN110405933A (zh) * 2019-08-31 2019-11-05 乌鲁木齐益好天成新型节能材料有限公司 微波、近红外双辐射sg外墙防火保温板生产流水线
CN114486988B (zh) * 2022-01-27 2024-03-29 东北大学 一种真空环境下微波移动烧结月壤试验装置及试验方法

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