Innovative medical devices are often made possible by new technologies emerging from component suppliers. In the seemingly staid field of electric heat, one such development is the use of advanced ceramic heaters that offer the potential to help with size and weight constraints, performance improvements, and surprisingly, regulatory compliance.

Why advanced ceramics

While the attributes of ceramics are generally appreciated, the characteristics of ceramic heaters are foreign to most. Advanced ceramics are synthetically produced, high purity, inorganics such as alumina (Al₂O₃), zirconia (ZrO₂), silicon nitride (Si₃N₄), and aluminum nitride (AlN). This last material shows great promise.

Advanced ceramic materials all are unique with respect to their mechanical, thermal, and electrical properties. Depending on the application, these properties become the selection criteria to determine which advanced ceramic material is appropriate for a particular heating requirement.

Manufacturing advanced ceramics often involves coalescing the material at high pressure followed by heating, or sintering, at high temperatures. The controlled purity and chemistry of the material results in a monolithic, geometrically stable structure that is responsible for mechanical, electrical, and thermal properties that work well as heater substrates. In the case of AlN, the features include a high-dielectric strength, low leakage current, low coefficient of thermal expansion, high durability with low mass, and high thermal conductivity. The extremely dense microstructure eliminates moisture ingress and subsequent degradation of the dielectric — a useful attribute for many medical applications.

The use of AlN as the ceramic heater platform of choice is due to its excellent physical, electrical, and thermal properties such as:

• High thermal conductivity close to aluminum. Rapid heat dissipation allows constructing heaters with high watt densities enabling thermal ramp rates of up to 150 C degrees (270 F degrees) per second.

• The clean, noncontaminating material comes from high temperature sintering that produces a heater with a Vickers hardness of 1,100 kg/mm² , and density of 3.2 g/cm³ with almost no porosity or surface roughness. This means AlN is an ideal choice for applications requiring a “clean” heater.

• The moisture resistance of AlN makes it impervious to liquids. This is unlike many hydroscopic dielectric materials in conventional heaters.

• The high dielectric strength and high insulation resistance makes AlN an electrical insulator with a low leakage current of less than 100µA at 500 Vac, a useful characteristic for many applications.

High-performance ceramic heaters come with power densities up to 1,000 W/in² and operate at 600C (1,112F) depending on application, heater design, and process parameters. An integrated thermocouple configuration improves the reliability of the sensor-heater interface to ensure control responsiveness during high-ramp rates. With customization easily available, these products can be machined into complex geometries including rings or crescent shapes. In addition, surface features such as fluid channels, contours, counter bored holes or other topographies of interest can be constructed to meet specific and unique design requirements.

Natural fits for advanced ceramic heaters are those that value characteristics such as ramp rate, uniformity, low-leakage current, compact size, cleanliness and chemical compatibility. These characteristics offer significant opportunities for product improvement. Three important areas deserve attention: meeting ISO specs, small size, and packaging.

To meet IEC 60601 electrical requirements, system designers are often required to use expensive (and heavy) step-down transformers to meet the extremely low-leakage current required of equipment used close to patients. Engineering a global platform that accommodates all anticipated voltages can be particularly problematic especially for machines connected to patients such as respirators, vaporizers, dialysis equipment, or other. When the heating system is a major contributor to overall leakage current, the high insulation resistance of a heater made from AlN can ease overall electrical system design and cost challenges associated with meeting the requirements.

“High wattage in a small package” does not conjure images of spectacular innovation unless you consider that a 0.5-in. square device can deliver over 800 Watts. That's equivalent to over 1,600 W/in². Delivering this kind of power makes advanced ceramic heaters capable of heating to 200C in less than 0.5 seconds, improving devices whose performance relies on cycle time. Rapid thermal cycling, while useful for some surgical devices, is also typically associated with various protein denaturization techniques associated with clinical chemistry equipment.

A glass-like surface finish can be achieved by polishing the ceramic surface. The sintering process provides a highly dense structure with pore sizes in the micron range enabling a surface roughness as low as 0.1 µm. The combination of high thermal-ramp rates and a highly engineered surface finish give advanced ceramic heaters the potential for innovations not only to medical devices themselves, but to the packaging and assembly sides of device manufacturing.

But the old heater works fine

A more often asked design question today is: Why upgrade to a new device when the old one works fine? “If it is not broken, don't fix it” may be a reasonable product strategy.

But without realizing it, a legacy heater may contain hidden costs in assembly labor, reliability, or simply unrealized performance benefits. Should these potential improvements exist, then the benefits of advanced ceramic heaters may be worth exploring.