Many of today’s ceramic innovations would not have been possible if not for the development of advanced furnace designs.
Small electric furnace capable of precision firing with both convection and radiant heating, along with a thermal oxidizer.
To state that companies “fire” their nano-scale ceramic materials and devices is a gross over-simplification in today’s world of materials science. What we are able to do to alter the properties of inorganic nano-scale materials is equivalent to stem cell technology’s impact on biological research.
Current advanced work in ceramic membrane and fuel cell technology is testimony to the advancement of our ceramic science. Such innovations would not have been possible without the simultaneous development of advanced furnace designs capable of the world-class performance necessary to support this technology.
Chemical Reactions at High Temperatures
As recently as 40 years ago, it was sufficient to say that “we fired ceramics.” In fact, at that time, fired traditional ceramics (e.g., dinnerware, tile, sanitaryware, artware, etc.) were still fired in much the same way that potteries had done for the previous century.
Now consider the development of nano-structured materials and products. Doping, for example, is an important process in semiconductor production and it involves purposely introducing finite amounts of chosen impurities into an extremely pure material with the intent of changing its ceramic structure and consequently its electrical properties. If you fully immerse yourself in the processing requirements for a new furnace that is intended to process these very advanced new materials, you soon realize that today we are designing new furnaces that-in a literal sense-perform precise and controlled chemical reactions at high temperatures.
Furnaces that have been designed for the heat processing of novel ceramic materials, devices and products are quite unlike furnace designs of the past. Considering the novelty of such new products and the unique processes by which they must be heat processed, the challenge for a ceramic manufacturer is to identify a furnace manufacturer with the right business model and experience to understand the process requirements and design a furnace that will precisely fulfill those requirements.
The manufacturer of these advanced ceramic products and the furnace builder are best-served when the customer can be very specific and exactly define all of the required furnace parameters, including processing temperatures during the heating and cooling phases, temperature uniformity at different stages throughout the cycle, furnace pressure (if any), and the chemistry of the furnace atmosphere within the furnace (if important). (See sidebar for additional details.)
Figure 1. Large-capacity electric furnace with both convection and radiant heating, plus a thermal oxidizer. The furnace also features a special clean interior refractory insulation system for firing products that demand absolute cleanliness.
Furnaces for the processing of ceramic membranes and solid oxide fuel cell devices require the integration of complex process heating and control systems in order to operate as integrated heating and process control systems.* Furnace designs for such processes are further complicated by the addition of structural refractories to support and carry the load setting, which can often impede a more perfect (uniform) heat flow.
World-class furnaces for technical ceramic applications may have to be controlled to as close as ± 2°C uniformity over much of the heating and cooling cycle (e.g., ambient to 1180°C/2160°F) for both the convection and radiant heating phases. Achieving such accurate uniformity is all the more remarkable considering that the largest interior volume of a furnace capable of such precision performance can be as large as 200 ft3 (see Figure 1). The simultaneous and precise control of multiple furnace temperature control zones, as well as control of numerous other process variables within this furnace, would not be possible today without the recent development of powerful digital controllers like Honeywell’s model HC-900 loop and logic controller.
Figure 2. The binder burnout process.
Sintering processes may also be accompanied by the added complication of organic binder materials that must be carefully burned off and removed early in the heat processing cycle and subsequently abated by oxidation to satisfy environmental requirements. The auxiliary exhaust and heating system associated with the oxidizer must then be carefully integrated into the basic furnace design so its performance does not upset the precise temperature control of the furnace (see Figure 2). *Keith Co. has developed and patented one such furnace, called the ConRad (descriptive of its convection and radiant heating systems).
New Furnace Designs
For the aforementioned advanced ceramic processes, both large and small furnaces typically require long, slow convection heating with slow rates of temperature rise (as slow as 1°C/hr) and no deviation from the ramp rate greater than 2°C, with ± 6°C temperature uniformity throughout the furnace load setting. If required, oxidation may have to occur simultaneously until a temperature of 500-600°C is reached and binder burnout is completed.
Meanwhile, heating during this low-temperature phase is achieved almost exclusively by convection with a high rate of circulation of the atmosphere within the furnace. It is with very specific and exacting process requirements such as these that a suitably qualified and experienced engineering team designing the furnace can complement the customers’ material science team to develop breakthrough furnace designs.
Just as planned temperature-dependent phase transformations are supposed to occur within materials during heating/cooling, other transformations occur depending on the rate of temperature change. If important, these types of process requirements need to be written into the technical specification for the furnace.
As mentioned previously, doped semiconductor materials are among the most hyper-sensitive products being produced today. In addition to the control of the furnace’s temperature and atmospheric chemistry at various temperatures, the cleanliness of the furnace interior can be another important requirement for the new furnace design. Vapor deposition on a semiconductor is typically not successful if dirt is present on the surface of the semiconducting material. Therefore, in addition to the usual thermal insulation, the furnace design might require a barrier of clean, hard interior refractory to help create an ultra-clean furnace interior.
As knowledge and the need to process advanced materials has progressed technologically, the parallel development of digital process control technology has enabled furnace manufacturers to develop the furnaces capable of processing and producing these technical ceramic products. Digital process controllers facilitate the required precision and, more importantly, enable the multi-tasking of multiple process variables. Without the latest generation of digital process controllers, few (if any) of the precise furnace performance parameters mentioned in this article would be possible.
Experience counts when it comes to developing new advanced ceramic materials and electro-ceramic devices. A well-matched combination of knowledgeable materials science design, together with an experienced furnace and controls design team, will provide the most successful path to continued, rapid and successful commercial development of ceramic science technologies and products. For more information, contact Keith Co. at 8323 Loch Lomond Dr., Pico Rivera, CA 90660; (562) 948-3636; fax (562) 949-3696; e-mail firstname.lastname@example.org; or visit www.keithcompany.com.
SIDEBAR: Furnace Performance Requirements
The design of a new furnace for the processing of oxide ceramics requires the consideration of many process variables, including:
- Processing temperatures (heating and cooling cycles)
- Temperature uniformity
- Temperature rise/cooling rates
- Multiple gas atmospheres
- Avoidance of certain chemicals within the furnace that can act as highly reactive contaminants
- Thermal oxidation