GB1597701A - Material with a honeycomb construction which absorbs electromagnetic waves - Google Patents

Description

(54) MATERIAL WITH A HONEYCOMB CONSTRUCTION WHICH ABSORBS ELECTROMAGNETIC WAVES (71) We, THOMSON - CFS, a French Body Corporate, of 173, Boulevard Haussmann 75008 Paris - France 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: The invention has for its object materials having a honeycomb structure of the "foam" type, formed by a plastics compound loaded during manufacture with a powder which absorbs electromagnetic waves especially microwaves.

It is known that materials exist which have absorbent properties, in this or that frequency range, for electromagnetic waves, because they give dielectric or magnetic losses in the presence of electrical and magnetic fields. In order to damp electromagnetic waves, a certain quantity of the absorbent material is arranged in a given volume; assuming, as is often the case, that there is no narrow limit on any dimension, an ideal material may be imagined which would have: good absorbing power (over a wide range of frequencies), - low density (a weight limit is desirable for some applications), - low hardness (for ease in machining), good resistance to mechanical stresses.

It can be seen that this list eliminates the classical ferromagnetic materials, in spite of their good absorbing power, because of their density and hardness.

On the other hand, synthetic resins loaded with absorbent marerial are easier to machine but their density is not negligible and they are less efficient than ferrites.

However, in the case of "foam" type products containing cells filled with gas, the density is much less than unity. The efficiency is a function of the quantity and absorbing power of the load accepted by the "foam" product without a degradation in its specific properties (low density due to the expansion of the material, ridigity). Activated charcoal has been used as load and a certain absorption of the electrical field has been noticed but it does not absorb the magnetic field.

The invention makes it possible to correct certain of the difficulties mentioned and, in particular, to reconcile the efficiency with the low density and hardness which are characteristic of "foam" products.

The material with a honeycomb construction of the "foam" type according to the invention is formed by a polyurethane plastics compound loaded during manufacture with a magnetic material having absorbent properties for electromagnetic waves.

This material may be prepared in the following way for example: - the basic constituents of polyurethane (glycol polyether and hexamethylene diisocyanate) are weighed; their weights should be the same. Magnetic oxide of iron (magnetite) in powder form is weighed in the proportion desired (usually from 40% to 60% by weight; - the magnetic iron oxide powder is added to one of the constituents by prolonged mixing until a mxiture with as homogeneous an appearance as possible is obtained; - the two constituents are mixed and shaken vigorously for instance using a screw propeller, for at least 20 seconds - the paste thus produced is put into moulds and left free to expand under the action of the gases produced by the mutual reaction of the polyurethane's constituents;; - after formation of the "foam", the material is left for some hours and then the product, which has become solid, is cut up for use.

The invention and its advantages will be better understood from the following examples: First example: 40% by weight of magnetic iron oxide of chemical formula Fe3 04.

This oxide is characterized by a magnetic moment of 61 U.E.M.C.G.S. per gram at ambient temperature. The mean density of the obtained material is 0.08 g/c.c. but it may vary between 0.03 and 0.20 g/c.c. depending on the intensity of the gas liberation during the reaction between the constituents.

From the material obtained, a parallele pipedic piece was cut out and slid inside a rectangular wave-guide so as to fill the inside volume completely for a length of 30 cm.

Electromagnetic radiation at 9 GHz (X band) was produced in the wave-guide and a power attentuation of about 4 decibels was measured across the absorbing substance. The energy reflected by the material was at a power level 23 dB below that of the incident radiation, the guide being terminated in a matched load.

Second example: 60% by weight of the same type of magnetic iron oxide as in the first example.

This quantity represents roughly the permissible upper limit if a product with good mechanical resistance is required. The density obtained is 0.138 g/c.c.

A test carried out under the same conditions as for the first example gave the following results: - attenuation 8.1 decibels, - reflected energy level ) 16 dB.

Third example: 50% by weight of magnetic iron oxide.

This oxide has a chemical formula close to Fe3 04 and a magnetization of 91 U.E.M.C.G.S.

per gram at ambient temperatures. It was prepared as follows: reduction at 5000 C for 6 hours of (Fe2 03) in a gaseous mixture formed by hydrogen filled with water vapour due to bubbling through water at 100"C.

The density of the loaded foam is 0.135 g/c.c.

Under the same conditions as in the first example, an attenuation of 10 decibels and a reflected energy level of - 17 decibels were obtained.

Among the other advantages of the invention the ease of use can be cited, for example: - for filling a given space in a waveguide system; - for lining an enclosure in which there is an electromagnetic radiation emitter by gluing sheets of the absorbent material in place.

The invention can also be used for protecting objects (screening) against electromagnetic radiation external to them.

WHAT WE CLAIM IS: 1. A material with a honeycomb construction of the "foam" type, formed by a polyurethane plastics compound loaded during manufacture with a magnetic material having absorbent properties for electromagnetic waves.

2. A material as claimed in claim 1, wherein said magnetic material is the magnetic oxide of iron (magnetite) incorporated in the polyurethane to the extent of 40% to 60% by weight.

3. A method of manufacture of the material claimed in claim 2, including the following steps: - addition, in the proportions given by weight of the magnetite in powder form to one of the constituents of the polyurethane (glycol polyether and hexamethylene diisocyanate); - mixing of the two constituents, thus form ing the "foam" under the action of the gases released by the reaction of the constituents; - cutting up of the product obtained for later use.

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

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. pipedic piece was cut out and slid inside a rectangular wave-guide so as to fill the inside volume completely for a length of 30 cm. Electromagnetic radiation at 9 GHz (X band) was produced in the wave-guide and a power attentuation of about 4 decibels was measured across the absorbing substance. The energy reflected by the material was at a power level 23 dB below that of the incident radiation, the guide being terminated in a matched load. Second example: 60% by weight of the same type of magnetic iron oxide as in the first example. This quantity represents roughly the permissible upper limit if a product with good mechanical resistance is required. The density obtained is 0.138 g/c.c. A test carried out under the same conditions as for the first example gave the following results: - attenuation 8.1 decibels, - reflected energy level ) 16 dB. Third example: 50% by weight of magnetic iron oxide. This oxide has a chemical formula close to Fe3 04 and a magnetization of 91 U.E.M.C.G.S. per gram at ambient temperatures. It was prepared as follows: reduction at 5000 C for 6 hours of (Fe2 03) in a gaseous mixture formed by hydrogen filled with water vapour due to bubbling through water at 100"C. The density of the loaded foam is 0.135 g/c.c. Under the same conditions as in the first example, an attenuation of 10 decibels and a reflected energy level of - 17 decibels were obtained. Among the other advantages of the invention the ease of use can be cited, for example: - for filling a given space in a waveguide system; - for lining an enclosure in which there is an electromagnetic radiation emitter by gluing sheets of the absorbent material in place. The invention can also be used for protecting objects (screening) against electromagnetic radiation external to them. WHAT WE CLAIM IS:

1. A material with a honeycomb construction of the "foam" type, formed by a polyurethane plastics compound loaded during manufacture with a magnetic material having absorbent properties for electromagnetic waves.

2. A material as claimed in claim 1, wherein said magnetic material is the magnetic oxide of iron (magnetite) incorporated in the polyurethane to the extent of 40% to 60% by weight.

3. A method of manufacture of the material claimed in claim 2, including the following steps: - addition, in the proportions given by weight of the magnetite in powder form to one of the constituents of the polyurethane (glycol polyether and hexamethylene diisocyanate); - mixing of the two constituents, thus form ing the "foam" under the action of the gases released by the reaction of the constituents; - cutting up of the product obtained for later use.