GB1597998A - Method of the susceptibility of a material to microwave energy heating - Google Patents

Description

(544 A METHOD OF IMPROVING THE SUSCEPTIBILITY\ OF A MATERIAL TO MICROWAVE ENERGY HEATING (71) We, SPECIAL METALS CORPORA TION, of Middle Settlement Road, New Hartford, New York 13413, United States of America, a Company organised and existing under the laws of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of improving the susceptibility of a material to microwave energy reaction and more particularly to such a method which enables granular materials which could not formerly be heated by microwaves to be so heated.

Microwaves are electromagnetic waves which transmit energy through space. The frequency spectrum which can be used for the microwave heating of an object is higher than radio waves and much lower in frequency than infrared or visible light. For example, for industrial and home microwave ovens, the F.C.C. allows four frequencies, the most common being 2450 MHz (2450 million cycles per second) because it generally permits the use of system components of reasonable size. The United States Government also permits use of 915 MHz. The effect of these microwaves on various materials vary. Materials can be classified into three general classes with respect to behaviour when exposed to microwave energy. These are (1) reflective, (2) transparent and (3) absorbant.

Metals and some other materials which fall into the first class act somewhat as a mirror and reflect the microwaves. Materials which act somewhat as glass and are transparent to the microwaves (microwave energy) so that they can pass therethrough fall in the second class.

Still other materials which are absorbant to the microwaves fall in the third class. Few materials are completely transparent or absorbant so that in most cases materials of the second class will absorb some microwave energy and materials of the third class will not absorb all the microwave energy. However, it is generally considered that materials which can be heated a substantial amount by microwaves fall into the third class. The heating rate is influenced by mass, weight, shape and composition.

For various reasons it may be desirable to heat materials of the first two classes set forth already by microwaves, but to the best of our knowledge this has not been possible up to the time of our invention.

We have found that materials which are not inftinsioally susceptible to heating by means of microwave energy can be rendeIed susceptible to heating by microwave energy when mixed in granular form with a reactive agent, as set out more fully below. For the purpose of identification herein, a reactive reagent will be defined as a compound which when mixed with another material in granular form will render the other material susceptible to heating by microwave energy radiation; a material which is readily heated by means of microwave energy (class 3) will be considered to be a material which "strongly couples" to microwave energy; a material which cannot be heated by microwave energy (classes 1 and 2) will be defined as a material which is "non-coupling" to microwave energy radiation; and those materials which can be heated by means of microwave energy radiation, but which do not heat too well or for which the temperature cannot be raised very much, will be designated as a "weak-coupler".

These are those which fall in the grey area between classes 2 and 3. We have found that those materials which act as "strong-couplers" to microwave energization when mixed with a material which is a "non-coupler" or "weakcoupler" will render the entire mixture readily susceptible to heating by means of microwave energy.

It is therefore an object of our invention to provide a method of improving the susceptibility of a material to microwave energy so as to render it readily heatable to high temperatures by means of microwave energy radiation.

The present invention provides a method of improving the susceptibility of a substance to heating to high temperatures by microwave energy reaction, which comprises mixing said substance in granular form with a microwave reactive reagent in granular form, which reagent is capable of substantially greater response to microwave energy than said substance, whereby the entire mixture may be heated by microwave energy.

The present invention further provides a method of making an article which is susceptible to heating to high temperatures by microwave energy reaction, which comprises mixing together a major proportion of Awl 203 iin granular form with a minor proportion of CaO in granular form to form a dry mixture, then adding a minor proportion of water to said dry mixture to form a wet mixture in which the ingredients react, and forming said wet mixture into said susceptible article.

According to our invention we improve the susceptibility of a material to microwave energy reaction by associating it with a strong coupler. We have found by testing in granular form that the following materials are strong couplers: TABLE 1 TABLEI Cobalt Oxide -Co304 Chromium Nitride - CrN Iron Oxide (black) - Foe3 04 Manganese Dioxide - MnOz Nickel Oxide - NiO Nickel Oxide -Ni203 Calcium Aluminate - Canal yOz It is immaterial whether the above compounds are added as such or are formed during processing. For example, Fe203 which is not a strong coupler may be used and converted to Fe304 during processing.

By tests we have also found that a substance susceptible to microwave energy reaction can be made by mixing and reacting a major proportion of A1203 with a minor proportion of CaO and water. A1203 or CaO separately are poor couplers. The A1203 and CaO are mixed in granular form, water added to this dry mixture and reacted to obtain calcium aluminate.

The A1203 is preferably at least 90% by weight of the dry mixture and the water added to the dry mixture is preferably 8 to 15% by weight of the dry mixture. Commercial dry mixtures containing approximately 95% A1203 have been successfully used. One such mixture is sold under the trade name of Taylors' 440 Castable Refractory. This subsequent mixture now contains a desirable quantity of calcium aluminate and coupling occurs. Even after all or part of the water is removed the mixture remains a strong coupler either as a solid body or after being broken up into powder form.

According to one embodiment of our invention a commercial refractory material, such as MgO, which alone cannot be heated by nucro- wave energy is associated with one or more of the above strong couplers by mixing them in granular form. The entire mixture can then be heated by microwave energy to a high temperature, such as 25000F. Water may be added to the mixture and the mixture formed into a solid mass and dried prior to heating by microwaving energy.

Performance of a method according to the present invention enables the mixtures to be heated to red heat or incandescence so that high refractory temperatures are obtainable.

The amount of strong coupler is not critical.

As little as 1% will enable some heating to take place. More than 90% should not be used because the characteristic of the original material would be lost. However, the greater the percentage of the strong coupler the greater will be the efficiency (percent of microwave energy absorbed) of coupling.

It will be understood that the strong coupler may be associated with the substance having little or no susceptibility to microwave energy by mixing with the substance ingredients which will react to form the strong coupler in the mixture.

WHAT WE CLAIM IS: 1. A method of improving the susceptibility of a substance to heating to high temperatures by microwave energy reaction, which comprises mixing said substance in granular form with a microwave reactive reagent in granular form, which reagent is capable of substantially greater response to microwave energy than said substance, whereby the entire mixture may be heated by microwave energy.

2. A method according to Claim 1 including the step of forming said mixture into an article which is susceptible to heating to high temperatures by microwave energy reaction.

3. A method according to Claim 1 or Claim 2, wherein said reactive reagent is cobalt oxide (Co304), chromium nitride (CrN), iron oxide (we3 04), manganese dioxide (Mn02), nickel oxide (NiO), nickel oxide (no203), the reaction product of a mixture of a major proportion of A1203 and a minor proportion of CaO mixed with water and dried, or mixtures thereof.

4. A method of making an article which is susceptible to heating to high temperatures by microwave energy reaction, which comprises mixing together a major proportion of A1203 in granular form with a minor proportion of CaO in granular form to form a dry mixture, then adding a minor proportion of water to said dry mixture to form a wet mixture in which the ingredients react, and forming said wet mixture into said susceptible article.

5. A method according to Claim 4 in which at least 90% by weight of Awl2 03 in granular form is mixed with CaO in granular form to form the dry mixture, and the weight of water added is 8 to 15% of the weight of the dry mixture.

6. A method according to either one of Claims 4 and 5 in which the water is subsequently removed from the mixture.

7. A method of improving the susceptibility of a substance to heating to high temperatures by microwave energy reaction, substantially as hereinbefore described.

8. A method of making an article which is susceptible to heating to high temperatures by microwave energy reaction, substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. together a major proportion of Awl 203 iin granular form with a minor proportion of CaO in granular form to form a dry mixture, then adding a minor proportion of water to said dry mixture to form a wet mixture in which the ingredients react, and forming said wet mixture into said susceptible article. According to our invention we improve the susceptibility of a material to microwave energy reaction by associating it with a strong coupler. We have found by testing in granular form that the following materials are strong couplers: TABLE 1 TABLEI Cobalt Oxide -Co304 Chromium Nitride - CrN Iron Oxide (black) - Foe3 04 Manganese Dioxide - MnOz Nickel Oxide - NiO Nickel Oxide -Ni203 Calcium Aluminate - Canal yOz It is immaterial whether the above compounds are added as such or are formed during processing. For example, Fe203 which is not a strong coupler may be used and converted to Fe304 during processing. By tests we have also found that a substance susceptible to microwave energy reaction can be made by mixing and reacting a major proportion of A1203 with a minor proportion of CaO and water. A1203 or CaO separately are poor couplers. The A1203 and CaO are mixed in granular form, water added to this dry mixture and reacted to obtain calcium aluminate. The A1203 is preferably at least 90% by weight of the dry mixture and the water added to the dry mixture is preferably 8 to 15% by weight of the dry mixture. Commercial dry mixtures containing approximately 95% A1203 have been successfully used. One such mixture is sold under the trade name of Taylors' 440 Castable Refractory. This subsequent mixture now contains a desirable quantity of calcium aluminate and coupling occurs. Even after all or part of the water is removed the mixture remains a strong coupler either as a solid body or after being broken up into powder form. According to one embodiment of our invention a commercial refractory material, such as MgO, which alone cannot be heated by nucro- wave energy is associated with one or more of the above strong couplers by mixing them in granular form. The entire mixture can then be heated by microwave energy to a high temperature, such as 25000F. Water may be added to the mixture and the mixture formed into a solid mass and dried prior to heating by microwaving energy. Performance of a method according to the present invention enables the mixtures to be heated to red heat or incandescence so that high refractory temperatures are obtainable. The amount of strong coupler is not critical. As little as 1% will enable some heating to take place. More than 90% should not be used because the characteristic of the original material would be lost. However, the greater the percentage of the strong coupler the greater will be the efficiency (percent of microwave energy absorbed) of coupling. It will be understood that the strong coupler may be associated with the substance having little or no susceptibility to microwave energy by mixing with the substance ingredients which will react to form the strong coupler in the mixture. WHAT WE CLAIM IS:

1. A method of improving the susceptibility of a substance to heating to high temperatures by microwave energy reaction, which comprises mixing said substance in granular form with a microwave reactive reagent in granular form, which reagent is capable of substantially greater response to microwave energy than said substance, whereby the entire mixture may be heated by microwave energy.

2. A method according to Claim 1 including the step of forming said mixture into an article which is susceptible to heating to high temperatures by microwave energy reaction.

3. A method according to Claim 1 or Claim 2, wherein said reactive reagent is cobalt oxide (Co304), chromium nitride (CrN), iron oxide (we3 04), manganese dioxide (Mn02), nickel oxide (NiO), nickel oxide (no203), the reaction product of a mixture of a major proportion of A1203 and a minor proportion of CaO mixed with water and dried, or mixtures thereof.

4. A method of making an article which is susceptible to heating to high temperatures by microwave energy reaction, which comprises mixing together a major proportion of A1203 in granular form with a minor proportion of CaO in granular form to form a dry mixture, then adding a minor proportion of water to said dry mixture to form a wet mixture in which the ingredients react, and forming said wet mixture into said susceptible article.

5. A method according to Claim 4 in which at least 90% by weight of Awl2 03 in granular form is mixed with CaO in granular form to form the dry mixture, and the weight of water added is 8 to 15% of the weight of the dry mixture.

6. A method according to either one of Claims 4 and 5 in which the water is subsequently removed from the mixture.

7. A method of improving the susceptibility of a substance to heating to high temperatures by microwave energy reaction, substantially as hereinbefore described.

8. A method of making an article which is susceptible to heating to high temperatures by microwave energy reaction, substantially as hereinbefore described.