Srinivasan Sridharan, is group leader, Ferro Electronic Material Systems, Independence, Ohio; Orville W. Brown is senior research scientist, Ferro Electronic Material Systems, Vista, Calif.; Keith Mason is global marketing manager, Ferro Electronic Material Systems, Vista, Calif.; and Pieter Dijkstra is international business manager, Ferro Industrial Coatings Group, Rotterdam, The Netherlands.

Thick-film heaters embedded on an oven’s enamel coating have been of interest to the appliance industry as a means of reducing energy consumption, creating a compact look, and facilitating easier cleaning of spillage. However, designers’ attempts to adopt this technology have been blocked by two major obstacles. One is achievement of the desired electrical characteristics at an acceptable oven operating temperature without cracking in the enamel layer, and the other is avoidance of premature breakdown voltage (BDV) failures.

An environmentally friendly thick-film material system of compatible enamels, resistors, conductors, and overglazes, as well as deposition methods have been developed that solve these technical issues and fire in conventional porcelain enamel fast furnaces. This technology is gaining traction, and a working concept of a thick film household oven has been demonstrated that meets the following objectives:

• Fulfills the electrical safety and long-term reliability requirements for stationary heating appliances.
• Meets the energy standards of class A ovens.
• Provides good to very good baking results.

Material developments

The materials combination in a thick-film heater typically include a thin-gage low-carbon steel substrate, enamels that provide electrical insulation, a resistive heating film, and an overglaze layer to protect the resistor traces. The enamel layer in a film-heated oven panel electrically separates the metal substrate and the resistor layer, thus its electrical properties — particularly Insulation Resistance (IR), dielectric constant (K), and loss factor — govern whether the panel’s electrical performance meets requirements for low leakage current (LC) and high BDV. The variations of these electrical properties with temperature determine the useful upper operating temperature of the film-heated oven panel. The thickness of the enamel layer and the distribution and sizes of the pores in it also play important roles.

Conventional porcelain enamels for low-carbon steel substrates are generally based on alkali borosilicate glasses. These glasses fire at a typical peak temperature of 780 DegC to 850 DegC with about five minutes above 700 DegC, in a continuous fast belt furnace with a total in-and-out time of about 20 min. Due to excessive alkali ions in these enamels, their insulation resistance quickly degrades with a rise in temperature, especially in the 200 DegC to 300 DegC range. As a result, they are electrically conductive for all practical purposes at the normal oven operating temperatures of 200 DegC to 350 DegC.

A significant part of the solution to this safety problem was the development of a series of alkali-free glasses for enamel applications that fire in typical porcelain enamel furnace firing conditions (Ref. 1), as described above. As shown in Fig. 1 (Ref.2), the IR of these “low-leakage” enamels is about four to five orders of magnitude higher than that of the conventional enamels in the 200 DegC to 350 DegC region. Further, these new enamels exhibit a leakage current-to-power ratio of about 0.12 mA/kW, or less, at 300 DegC, for a typical enamel fired thickness of 200 microns to 250 microns.

Deposition optimization

The electrostatic spray process typically used to apply porcelain enamel to oven-cavity panels also contributed to the formation of large bubbles that were difficult to control. In addition, it did not permit adequate control of deposition thickness, and its variations were found to be a major cause of cracking in the enamel. The exploration of alternative techniques of enamel application found that screen printing yielded the best results in both controlling variations and in reducing overall enamel thickness.

New dielectric enamel screen-printing pastes were developed that promote excellent bonding and adhesion to the low carbon steel substrate without generating a significant amount of bubbles at the interface, as shown in Fig. 2 (Ref. 2). In addition, their thermal expansion better matches that of low-carbon steel to reduce warping and cracking.

These microstructures passed the critical requirement of BDV>1,250 VAC for 60 sec. at both 25 DegC and 350 DegC without failure for the dielectric thickness of about 125 microns. These dielectrics exhibit very low leakage up to 400 DegC to 450 DegC and breakdown occurs only at temperatures greater than about 500 DegC during testing. By comparison, conventional enamels fail this test even at room temperature.

RoHS-compliant lead-free and cadmium-free resistors and overglazes were developed that exhibit the excellent compatibility shown in Fig. 3 (Ref. 2). Characteristics of the resistor paste used to develop the oven panel include positive Temperature Coefficient of Resistance (TCR) behavior, sheet resistance of 10 milliohm/sq. at room temperature, TCR of 3,100 ppm/DegC, and firing temperature of 630 DegC to 700 DegC for 10 min. The overglaze was designed to minimize any resistance shift due to overglaze/resistor reaction during either firing (610 DegC ~ 650 DegC) or subsequent lifetime testing (Ref. 3).

This research was conducted jointly with Electrolux Major Appliances, Rothenburg o.d.T, Germany, which designed the thick-film heater (TFH) panel which, built the prototype oven and conducted oven performance testing. Curvatures in the embossed cavity panels were found to contribute to enamel cracking along with variations in the enamel thickness, so the panel design was changed to a flat configuration.

A proprietary heater pattern was developed to optimize heat distribution over the entire panel surface, and further refined to minimize the temperature gradient. The resulting pattern was screen-printed using the resistor paste described above on the flat low carbon steel substrate and fired, then its performance was monitored in real time with an infrared camera. The even temperature distribution is illustrated in Fig. 4 (Ref. 2).

The main achievement in this project was that the TFH panels passed the electrical safety requirements for Stationary Class I heating appliances, according to IEC 60335-1 (Ref. 4). The leakage current was well within the safety limit 0.75 mA per applied kW, and the panels passed the break down test at operating temperatures (60 sec. of applied 1,250 VAC at about 300 DegC) (Ref. 4, 5).

The heat-up rate for the TFH panels was about 8 min. to 10 min. to reach the steady state operating temperature of 300 DegC ~ 320 DegC. A lifetime endurance test was performed according to Electrolux’ internal standard for tubular heating elements. The test required a minimum of eight panels to be run at operating temperature while cycling 45 min. ON and 15 min OFF until 2,000 ON-hours are reached without failure. After passing the electrical safety tests, nine prototype panels were subjected to the lifetime test. Of these, seven passed the test, with a change in resistance that did not exceed ±5 percent. Two panels failed for reasons unrelated to the lifetime testing (Ref. 2).

Oven performance

The working concept oven implemented the flat TFH panel as a bottom heating element and used a non-modified top conventional tubular heating element. The panel directly heated the cavity bottom, and energy consumption for the prototype oven was below 800 Wh, which is equivalent to or better than energy Class A for its cavity size (Ref. 6). Results of cooking tests to compare food quality shown in Fig. 5 (Ref. 2) demonstrate that for most of the main standardized test recipes, the TFH panel is a comparable replacement for the bottom conventional tubular heating element.

In addition to energy efficiency, the TFH bottom panel’s maximum temperature did not exceed 300 DegC, versus 400 DegC or more for the bottom cavity of a conventionally heated oven (Ref. 2). Thus, this reliable and reproducible prototype oven exhibits easier cleaning of burnt spillage and good to very good baking results.

Near commercialization

Although this particular TFH oven project has not proceeded further as of this time, several appliance manufacturers are evaluating the technology for flat panel applications, including ovens, in-line water heaters, small appliances, and industrial heaters.

For more information email: masonk@ferro.com

References:

1. Sridharan S., et al., “Porcelain Enamel Composition for Electronic Applications,” U.S. Patent 5,998,037, Dec 7, 1999.

2. Sridharan S., et al., “Thick Film Heated Oven With Low Energy Consumption,” 20th International Enamellers Congress, May 15-19, 2005, Istanbul, Turkey.

3. Sridharan S., et al., “Electronic Device having Lead free and Cadmium free Electronic Overglaze Applied Thereof,” U.S. Patent Pub. No. US2004/0018931A1, Jan 29, 2004.

4. IEC 60335-1 International Standard, “Household and similar electrical appliances – Safety – Part 1: “General requirements,” 2001.

5. IEC 60335-2–6 International Standard, “Household and similar electrical appliances – Safety – Part 2–6: Particular requirements for stationary cooking ranges, hobs, ovens and similar appliances,” 2002.

6. EN 50304 European Standard, “Electrical ovens for household use – methods for measuring the energy consumption,” 2001.