A novel, thick-film heating element technology has been engineered to create a heat source that offers a uniform temperature gradient across the element surface, reduced heat-up and cool down cycles while maintaining a cost benefit over conventional heating element technologies. The technology can be used to manufacture cost effective, low-profile heaters for commercial and industrial applications, and is well suited to serve as an alternative heat source for warming applications.

Datec Coating Corp. Mississauga, Ontario, developed the new thick-film technology using its method for developing ceramic coatings. In the Datec process, a ceramic coating is applied to a mica insulation board, and then fired at a temperature as low as 350 DegC to form a heating element.

Thick-film heaters have been of interest for commercial and industrial applications because they provide a number of advantages over conventional metal-sheathed resistance elements. Thick-film heaters have a low profile, fast thermal response, improved temperature uniformity over large surfaces, and higher operating efficiency.

However, they are not without limitations: until now “traditional” thick-film heaters have had high materials costs, complex processing schedules, high scrap rates, and have been limited to glass-based enamels. In addition, high processing temperatures, in the range of 650 DegC to 850 DegC, restrict their use to those substrates that can withstand the high processing temperatures and have a similar coefficient of thermal expansion.

By contrast, Datec’s process is an engineered technology that overcomes the limitations of the traditional thick-film heaters and provides a cost effective, easy to manufacture and environmentally friendly solution. With this process, the ceramic coating system can be fired at low temperatures, thus increasing the range of potential substrates to include glass, mica, polyimide, quartz, stainless steels, aluminum and super alloys. In addition, Datec’s thick films can be applied over large surface areas providing excellent thermal uniformity that, combined with a high emissivity of 0.95, make the system suitable for any heating application requiring efficient radiant heat transfer.

Fig. 2. IR images showing the temperature profile of a conventional heater (left) and a Datec CSG heater (right) operating at 100 W

The composite sol-gel coating (CSG) process is an innovative method developed and patented1 by Datec. In this process, fine powders of ceramics, metals and polymers are dispersed in a sol-gel solution to form a sol-gel paste that can be sprayed, dipped, screen-printed or spin-deposited on a substrate. The combination of selected powders and sol-gel binders creates a coating system with the properties required for numerous applications.

Once fired, the sol-gel phase acts to bind the powder phase and adheres the overall coating to the substrate. This method combines the advantageous properties of conventional sol-gel with the ability to produce much thicker (up to 500 microns), adherent coatings through control of the coating microstructure. The beneficial properties of the ceramic powder obtained through high-temperature processing (>1000 DegC) are incorporated in a composite coating that may be processed in air at temperatures as low as 350 DegC.

This CSG technology has been used by Datec to engineer coatings for many applications, including corrosion protection, thermal barriers, optical coatings, resistive coatings, conductive coatings, and dielectric coatings.

The CSG resistive, conductive and dielectric pastes are suitable for making heaters on various substrates, including aluminum, ferritic and austenitic stainless steels, quartz, glass and ceramics. For metal heater applications, such as kettles, a CSG dielectric paste with a high breakdown voltage has been developed that is compatible with Datec’s CSG resistive and conductive coatings. Mica makes an excellent substrate because it is thin and light, has excellent dielectric strength and high temperature capability and form basis for a class of heater materials.

The process for mica heaters combines a CSG thick film with a thin-gage mica substrate that provides excellent electrical and thermal insulation as well high temperature withstand. Datec’s patented2 resistive and conductive CSG thick films are formulated for screen-printing and to promote excellent bonding and adhesion to mica. They can be deposited with a resistance tolerance of +/-5 percent. The pastes are water-based and RoHS-compliant, being free of both lead and cadmium.

Enlarge this picture Fig. 3. Graph showing the thermal response for conventional and Datec CSG heater.

To make the Datec thick-film heater, a layer of conductive paste is first deposited to form a bus bar network for the electrical circuit. A second layer consisting of the resistive paste is then deposited over this electric circuit. The coated substrate is then fired causing the sol-gel to bind the powder and adhere the overall coating to the substrate. A topcoat is applied to protect the heating layer.

The electrical resistivity of mica heaters can be varied in a range from 2 Ω/□to 100 Ω/□ allowing for significant flexibility in the heater design. A power density of approximately 1.2 W/cm2 is required to achieve a surface temperature of 250 DegC when the element is operated in air. Unlike metallic heating elements that have a positive temperature coefficient of resistance (TCR), the CSG resistor has a slightly negative TCR, in the range of –0.08 percent per DegC. The low thermal mass of the mica allows for a good thermal response so that the heater heats up and cools down faster than a conventional metallic heater element.

The large areas of deposition, up to 1 m by 0.75 m, and area coverage greater than 75 percent, provide excellent thermal uniformity and radiant heat transfer. In addition, the geometry of the heaters can be manipulated and watt density varied in different zones to compensate for non-uniform heat loss and edge effect, resulting in improved temperature uniformity. A selection of flat, two-dimensional heaters is shown in Fig. 1. However, the flexible application methods for CSG can be adapted to deposition on substrates with cylindrical and curved geometries.

Because the heat is distributed over a large area, this provides the additional advantage --  the surface temperature anywhere on the heater remains low, much lower than the maximum temperature of conventional metal-sheathed, etched-foil of “traditional” thick-film resistance elements with the same power rating. At maximum power density, CSG mica heaters operate at temperatures below 250 DegC when heated in air. When the elements are encased, the watt density can be increased or reduced, depending on the heat sink adjacent to the element.

The material properties and performance characteristics of mica heaters, which have CUR approval, create opportunities for numerous residential and commercial heating applications including:

• Space heaters.
  
• Warming plates.
  
• Decorative heaters.
  
• Ceiling heaters.
  
• Commercial foodservice appliances.

The advantages offered by the mica heater, including excellent temperature uniformity, makes it an obvious replacement for conventional heaters for stove-warming applications.

Fig. 2 shows infrared images of a mica heater and traditional metal element heaters for a 100 W stove-warming application. Both images were taken on the glass ceramic stovetop in the same time lapse. The conventional heating coil provides heat only along narrow bands, producing hot and cool spots throughout the area, as seen in the thermal image. The mica heater has a more uniform temperature over the entire surface producing more distributed, even heating.

The low thermal mass of the heater also improves the thermal response, so that the heater heats up faster than a conventional heater element. Fig. 3 shows the time-to-temperature curves as measured on the surface of the glass ceramic for a mica heater and conventional heater element at 180 W. The heat up time for the mica heater is faster.

The Datec process that utilizes a new ceramic coating technology has been proven as a commercially viable, cost-effective alternative to conventional, heating-element designs. The properties and performance characteristics of these thick-film heating elements offer space savings, performance advantages and cost reduction design opportunities for design engineers in a wide range of warmer applications.

1. WO9629447, EP0815285 and US 5585136  “Method for producing thick ceramic films by a sol-gel coating process”; WO2004113255 (Application) and US2004258611 (Application) “A colloidal composite sol gel formulation with expanded gel network for making thick inorganic coatings.”
   
   
2. US 6736997 “Sol-gel derived Resistive and Conductive coating”; US Pub. Appl. No. 200502 05548 “Low-temperature-fired, lead-free, thick-film heating element.”