Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating inorganic material – not as pigments,...
Reexamination Certificate
2001-12-18
2004-03-16
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating inorganic material, not as pigments,...
C264S630000
Reexamination Certificate
active
06706233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for processing ceramic materials. In particular, this invention relates to a method for heating ceramic materials involving the use of electromagnetic energy and optionally firing ceramics involving the use of electromagnetic energy in combination with conventional radiative/convective heating, and more particularly to a method for controlling the power dispersion of the electromagnetic energy by density selection of the pieces to uniformly distribute the heating energy throughout the ceramic material.
2. Technical Background
Conventional heating used in the manufacturing of ceramic materials typically utilizes radiative gas or electric resistance heating. Utilization of conventional radiative/convective heating typically results in a thermal differential within the ceramic material. This differential is due, in part, to the fact that radiant heating is applied only to the surface of the material and it relies on the thermal conductivity of the material, typically poor, to transmit the thermal energy beneath the surface and into the interior or core of the piece. In other words, conventional heating involves heat transfer that is predominantly achieved by radiation or convection to the surface followed by conduction from the surface into the interior of the ceramic body. If a core-surface thermal differential develops that is too great, internal cracking, external cracking, and distortion of the ceramic material can occur. Fast drying or firing further exacerbates this problem of poor heat transfer, and ultimately cracking.
Additionally, the presence of a core-surface thermal gradient can also result in uneven sintering, specifically surface sintering prior to, and at a faster rate than, interior sintering. As a result, the ceramic material may exhibit non-uniform properties.
Solutions to these problems which have been proposed involve reducing the rate of heating or allowing lengthy holds at certain temperatures. Each of these solutions allows heat energy to be conducted into the core of the ceramic material, which in turn, allows the temperature of the core of the ceramic material to “catch up” with that of the surface, thereby minimizing the surface/core temperature differential. Unfortunately however, the theoretical limits of conventional radiative or convective heating typically result in slow heating rates for all ceramic materials, the exception being ceramic pieces exhibiting small dimensions.
Microwave heating of ceramics has alternatively been successfully used to both dry and assist in firing ceramic materials. In comparison with conventional heating, microwave heating involves depositing energy directly within the ceramic material in accordance with a volumetric heating mechanism. More specifically, the utilization of microwave energy involves delivering a uniform application of the energy to the entire cross section of the ceramic article, rather than to the article surface. Although microwave heating of ceramic materials is much faster than conventional radiant heating because of this volumetric heating, it, like radiative heating, results in the ceramic article exhibiting a thermal differential; albeit an opposite thermal differential with the core of the ceramic article exhibiting a higher temperature than that of the surface. Specifically, as the ceramic materials, typically poor absorbers of microwave energy at low to intermediate temperatures, are heated with microwaves at high temperatures, the interior of the ceramic article very rapidly begins to absorb substantial amounts of microwave energy; this effect is known as thermal runaway. Although the surface is heated along with the core of the ceramic material, the surface rapidly loses much of its heat energy to the surroundings, which is typically cooler than the average ceramic material temperature. As the core starts to preferentially absorb the microwave energy this thermal runaway phenomenon becomes self-propagating. Simply stated, as the temperature of the ceramic material increases, the heat losses become greater, and the magnitude of the core-surface thermal differential increases, again leading to thermal stress within, and ultimately cracking of, the ceramic article.
In addition to heat losses from the surface of the ceramic article, non-uniformity of the microwave distribution within the dryer, kiln, furnace, or oven, and non-uniform material properties of the ceramic article lead to differential absorption of the microwave energy by the ceramic article, and contribute to the microwave heating thermal differential.
In the processing of cellular ceramic products, the as-extruded piece is subjected to several steps in which the piece is dried and fired, separately. All steps have specific time-temperature cycles in which the heating rates, hold temperatures, and hold times are all important to the formation of the required physical properties of the body. Using conventional hot air techniques, it can take longer to produce relatively larger parts. Therefore, depending upon the size of the part substantial lead time may be required for delivery of a product in the best of circumstances.
In an effort to alleviate this concern prior methods include the use of a combination of microwave energy and conventional heating techniques (resistive, gas firing, etc.) to process cellular ceramics from extrusion through the firing using one thermal process. This includes drying and firing, and eliminates the handling step (or steps, where the parts are dried twice) between dry and fire. The process can be applied to other cellular ceramic products as well.
Hybrid microwave/conventional heating or microwave assisted heating has been utilized as an alternative to overcome the problems of conventional radiative and microwave-only heating. In microwave assisted heating involving both microwave and radiative/convective heating, the volumetric heating provided by the microwaves heats the components, while the conventional radiative/convective heating provided by gas flame or electric resistance heating elements minimizes heat loss from the surface of the components by providing heat to the surface and its surroundings. This combination or hybrid heating can result in heating that avoids thermal profiles associated with conventional and microwave-only heating. As a result, thermal stresses can be reduced and or minimized and thus the ceramic articles can be heated more rapidly.
Conventional dielectric drying processes and gas firing can be combined in one thermal process by using microwave energy to assist in drying and firing parts faster and with less handling. Microwave drying works on the same principle as do the dielectric dryers, but is of a higher frequency and can be run more efficiently. Microwave assisted firing can reduce thermal gradients through a part during firing, allowing faster heating ramps, usually cutting ramp times by 50% or more of conventional gas firing.
In drying a wet piece, volumetric heating specifically aimed at polar molecules (i.e., water) is a great advantage over conventional methods of drying. This is how current dryers work. The advantages of using microwave drying are two fold. The high frequency of microwave energy allows the use of lower wattage and more efficient drying, while the actual apparatus has a smaller footprint. Also, unlike dielectric dryers, a microwave energy source can be used to assist in firing ceramics. A thermal process set to dry and fire parts would require no handling from the dryer to the kiln, and no cooling and re-heating steps either.
While microwave energy alone can be used to heat cellular ceramics, a much more efficient and reliable method is to meld the current technology in gas fired kilns with microwave assisted heating, creating a hybrid kiln capable of fast firing. Green ware is made up of organic and inorganic materials, and they react in different ways as they are subjected to the time-temperature cycle of firing. The organic materials burn in the presence o
Araya Carlos R.
Brennan John H.
Squier Gary G.
Vileno Elizabeth M.
Wexell Kathleen A.
Corning Incorporated
Fiorilla Christopher A.
Kees van der Sterre
LandOfFree
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