Ceramic material revs up microwaving

August 28, 2008

Quicker microwave meals that use less energy may soon be possible with new ceramic microwave dishes and, according to the material scientists responsible, this same material could help with organic waste remediation.

"Currently, food heated in a microwave loses heat to the cold dish because the dishes are transparent to microwaves," says Sridhar Komarneni, distinguished professor of clay mineralogy, College of Agricultural Sciences at Penn State. "The plates are still cool when the cooking is completed."

Materials are transparent to microwaves because the microwaves do not interact with the molecules in standard tableware. With liquids like water, the microwaves cause the molecules to move back and forth creating heat.

Komarneni, working with Hiroaki Katsuki and Nobuaki Kamochi, Saga Ceramic Research Laboratory, Saga, Japan, developed a ceramic from petalite and magnetite sintered together that heats up in the microwave without causing equipment problems the way most metals do.

They report their material in a recent issue of Chemistry of Materials.

Petalite is a commonly occurring mineral that contains lithium, aluminum and silicon and is often used to make thermal shock resistant ceramics because it expands very little when heated. Ceramic sintering uses powdered minerals pressed together hard to form green bodies. These green objects are fired first at low and then high temperatures.

When the petalite and magnetite are fired together, the magnetite converts to an iron oxide that heats up when placed in a microwave.

A rice cooker made of this material cooked rice in half the time it normally takes in a non-heating microwave rice cooker.

"Rice cooks very well with these dishes," says Komarneni who is also a member of Penn State's Materials Research Institute. "Dishes heated by themselves or with food could keep the food hot of up to 15 minutes. One might even cook a pizza on a plate and then deliver it hot."

However, those accustomed to cooking in a microwave will need to remember that the plates are hot and will burn bare hands. Potholders are again necessary.

Food preparation applications abound. A company in Arita, Japan -- long a locus of ceramic manufacturing -- called Asahi Ceramics Research Company is manufacturing microwave ware.

The material's microwave heating properties suggest another use. Because the material expands very little when heated, the petalite magnetite material does not shatter under rapid microwave heating and cooling as other materials might. The researchers created a plate of the petalite magnetite ceramic and coated the solid plate structure with cooking oil. After heating for 120 seconds, 98 percent of the oil was gone, decomposed into its components.

"We used cooking oil because it is an innocuous substance," says Komarneni. "We could, perhaps, use this material in a closed system to decompose organic contaminants in soil or dirt."

The researchers believe that once optimized, the material could be used for a variety of remediation applications at a lower energy cost and with less residue than many current methods.

Source: Penn State

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not rated yet Aug 28, 2008
I believe microwaves could also benefit from a second microwave emitter that shoots the waves up and down instead of side to side.

Perhaps this is already a standard feature on high end microwaves? If not... Wow.
not rated yet Aug 28, 2008
"Currently, food heated in a microwave loses heat to the cold dish because the dishes are transparent to microwaves[...] The plates are still cool when the cooking is completed."

The fact that the dish is transparent to microwaves and still cold when removed from the microwave means that the microwaves heat the food instead of being wasted on heating the dish. If you want to increase the efficiency, reduce the heat conduction of the ceramic in the dish so that the thermal energy stays in the food instead of being lost as heat to the dish.
not rated yet Aug 28, 2008
I believe microwaves could also benefit from a second microwave emitter that shoots the waves up and down instead of side to side.

Perhaps this is already a standard feature on high end microwaves? If not... Wow.

Microwave ovens use a "fan" to disperse the microwaves and the inside is covered with metal which reflects the microwaves very effectively(they do not have a short enough wavelength to "see" the holes in the wiremesh). The microwaves bounce around in there until they are absorbed by something; much of it ends up heating your food, some ends up heating grease and crud in your microwave and some of it ends up heating the microwave oven itself.
not rated yet Aug 28, 2008
Ah, thanks, makes sense... In that case I would expect the higher end microwaves to more accurately disperse the waves. It's easy to tell with cheaper microwaves where the waves are canceling or simply not reaching their intended target before being absorbed.

While not surprising, I find it odd that all dishes could be completely transparent to microwaves. I could swear thinner/clearer glass dishes heat the contents more quickly than thicker opaque dishes.

This calls for experimentation!
not rated yet Aug 28, 2008
I've got stoneware plates and they get insanely hot in the microwave, which I personally think is a bad thing. Do we really need microwaves to be faster? I thought they were nice because only the food got hot?
not rated yet Aug 28, 2008
The transparency of conventional dishes would not mean that microwave energy was "not wasted" heating the dish but would instead mean that the microwaves which were not absorbed by the food leave through the dish. Keeping them there in the form of heat which is conducted back to the food (or industrially processed substance) is indeed an improvement as the empirical evidence demonstrates.
not rated yet Aug 28, 2008
The transparency of the dish to microwaves would mean that microwave energy is lost which this material would otherwise convert to heat and conduct into the food. I don't know if the Faraday cage of the oven works by absorbing the waves or reflecting them but even if it works by reflection the waves will not preferentially hit the food and will eventually leak out or be absorbed by water vapor. People with pacemakers may have less to worry about from modern microwaves but they still leak energy.

I suspect that industrial applications where people don't need to grab the dish right out of the oven will be more impacted by this discovery, especially where items or substances travel down a pipe or conveyor belt and a total Faraday cage is impractical.
not rated yet Aug 29, 2008

I think that the pacemaker problem actually comes from the circuit that energizes the magnetron (gigahertz range, thousands of volts). I don't believe microwaves penetrate far enough into the skin to have a small leakage do much of anything to a pacemaker. Just a guess though.

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