WO2002040799A1

WO2002040799A1 - An electromagnetic radiation shielding material

Info

Publication number: WO2002040799A1
Application number: PCT/GB2001/005009
Authority: WO
Inventors: John Griffiths
Original assignee: Mantle & Llay Limited
Priority date: 2000-11-14
Filing date: 2001-11-14
Publication date: 2002-05-23
Also published as: GB0027724D0; AU2002223797A1

Abstract

A shielding material comprises at least one basic building material and at least one shielding component capable of reflecting or absorbing electromagnetic radiation. The shielding material optionally includes a hydraulically settable binder or cementitions material. The shielding components may be natural materials or direct or derived by-products of an industrial process. The shielding component may comprise a combination of graphite, especially kish graphite, and carbon black, especially acetylene black.

Description

AN ELECTROMAGNETIC RADIATION SHIELDING MATERIAL

Introduction

The present invention relates to electromagnetic (EM) shielding materials for use in construction. More specifically the invention relates to the use of predominantly carbon-based and/or iron-based materials to impart shielding properties to a building construction.

Exposure to (EM) radiation or (EM) fields from, for example, mobile phones, telephone masts, power lines, airports and outside broadcast units has been suggested to cause a broad range of health problems. These range from an increased risk of developing cancer to depression, eye complaints, dizziness and heart complaints (Rutter T, BMJ 1998, 317; 12 (4 July) ) .

In addition the perceived threat from EM radiation or EM fields has led to a reduction in the prices of property in the USA sited nearby EM radiation sources. The growing public concern over the effects of EM radiation has also forced electrical companies to site power lines away from schools and dwellings. Further, EM radiation can disturb normal functioning of electrical and computer equipment.

Accordingly, there is a need to be able to protect people and/or equipment from EM radiation.

It is an object of the present invention to overcome at least some of the above problems. In one aspect, it is an object of the present invention to provide a shielding material that is ideally affordable and cost effective for use as a building material, but which provides adequate protection for use in shielding people or equipment from EM radiation.

Statements of Invention

According to the present invention there is provided a shielding material comprising one or more basic building materials, optionally including a hydraulically settable binder or cementitious material, and at least one shielding component capable of reflecting or absorbing EM radiation. Preferably, these components are direct or derived by-products of an industrial process.

In this specification, the term 'electromagnetic radiation' should be taken to mean that spectrum of electromagnetic radiation which has been suggested to cause health problems, and as such, should include electromagnetic radiation or fields emitted from mobile telephony masts, telephone masts, power lines (especially high voltage power lines) , airports and outside broadcoast units. Typically, the electromagnetic radiation will have a frequency in the Hz to GHz range .

The hydraulic binder is any material which when mixed with water sets and hardens to form a solid mass either at ambient conditions of temperature and humidity or at elevated temperatures and saturated conditions for example in an autoclave. Typical, although not exhaustive, examples include Portland Cement (PC), Calcium Aluminate cement, Calcium Sufoaluminate cement, Calcium Sulfate Hemihydrate (Plaster of Paris) and Calcium Sulfate Anhydrite

Combinations of PC with the following are also considered as hydraulic binders: Pozzolanic materials such as Pulverised Fuel Ash (PFA) , volcanic ash etc.; latent hydraulic materials such as Ground Granulated Blastfurnace Slag (GGBS) and other slags; finely ground silica; Microsilica; Metakaolin; Lime; Ground Limestone or other mixtures thereof.

Lime when mixed with fine materials containing silica, then cast and cured in an autoclave is also considered to be a hydraulic binder for the purposes of this invention. Typically the shielding components modify the overall electrical and/or magnetic properties of the materials to alter the reflectance and/or absorbance of the final product.

When a signal impacts on a surface, varying parts of the signal will be reflected, absorbed or transmitted. Very generally, materials that have high electrical conductivity also have good shielding ability in the MHz and GHz ranges, especially in reflection.

An electromagnetic signal consists of an electric and a magnetic component . At frequencies above about 20 Mhz, the electric field predominates and below this frequency, the magnetic field is more significant. Therefore, especially at low frequencies, superior shielding can be achieved if the magnetic component is also attenuated. This can be achieved by the inclusion of iron rich materials such as ferrite, which is costly. One aspect of this invention is the utilisation of components with an intrinsic iron content. These might include Magnetite, iron rich PFA, chopped steel fibre, steel swarf, natural graphites with high mineral impurities, and kish graphite (a by-product of steel making) . Many of these alternative shielding components are preferred as they are less costly than ferrite.

Materials capable of shielding at 50 Hz are suitable for providing protection against emissions from mains power lines . Materials which shield in the kHz range give protection from, for example, medium frequency broadcasting. Those that shield in the MHz range offer protection from, for example, emissions from telephone masts, and those that shield in the GHz range protect against emissions from civil aviation systems .

Preferably the shielding components which have reflective properties are chosen from the group consisting of; kish graphite, coke, exfoliated graphite, exfoliating graphite, artificial graphite (for example electrode graphite) , graphitised carbon fibres (pitch or polyacrylonitrile based, including scrap fibres) , natural graphite, flake graphite, mxllscale, steel fibre, steel swarf, non-graphitised carbon fibre, oxidised carbon fibre, carbon black (acetylene black or conductive blacks) and PFA. Generally, these materials are effective at shielding EM radiation in the kHz, MHz and GHz frequency ranges.

Preferably the shielding components which have absorptive properties are chosen from the group consisting of; carbon black such as lamp black, channel black, furnace black and thermal black, low rank coal charcoal, viscose based carbon fibre, carborundum, ferrite, iron oxide, magnetite and PFA. Generally, the above non-ferrous materials are effective at shielding EM radiation in the KHz to GHz frequency ranges. In certain circumstances, it may be useful to provide a laminate of shielding materials. Thus, for example, a laminate of shielding material may comprise an element which has good shielding properties in the medium to high frequency' range and an element which has good shielding properties in the low or low to medium range. Typically, the components of the laminate will depend to a large extent on which type of shielding is required. Thus, for example, a render or plaster may include laminates of shielding materials which are effective shields at both low (50Hz) and high (GHz) frequency ranges .

In one preferred embodiment of the invention, the component (s) capable of reflecting or absorbing radiation comprises a graphite and/or a carbon black. Typically, the graphite is kish graphite. Ideally, the carbon black is an acetylene black or a lamp black. In one embodiment of the invention, the graphite comprises between 0.5 and 60%, preferably between 10 and 40%, of the shielding material (w/w dry components) . Typically, the carbon black will comprise at least 0.5%, suitably between 1 and 20%, typically between 3 and 10% and preferably about 2 to 6%, of the shielding material (w/w dry components) .

In one particularly preferred embodiment of the invention, the component capable of reflecting or absorbing radiation comprises a combination of graphite, especially kish graphite, and carbon black, especially an acetylene black or a lamp black. Typically, lamp black will be preferred as it is presently less expensive than acetylene black. In such cases, the graphite will comprise between 0.5 and 50%, typically between 20 and 40%, of the shielding material (w/w dry components) . Typically in such cases, the carbon black will comprise between 0.5 and 10%, ideally between 1 and 5%, of the shielding material (w/w dry components) .

In a further embodiment of the invention, the at least one shielding component comprises a ferrous material. Typically, the ferrous material is magnetite which preferably comprises between 0.5 and 75% of the shielding material (w/w dry components) . Alternatively, the ferrous material is iron oxide which ideally comprises between 0.5 and 60% of the shielding material (w/w dry components) . Sμitably, the ferrous material is millscale which preferably comprises between 25 and 75% of the shielding material (w/w dry components) . In one embodiment of the invention, the shielding material may include a combination of two or more of the above ferrous material, optionally in combination with one or more of the non-ferrous shielding components.

The term 'building materials' as used in this specification should be taken to mean both formed building materials, such as, for example, bricks, blocks, slabs, cladding panels, tiles, roofing panels, isulating materials, cavity walls and fillings for cavity walls, floor blocks, screed, plasterboard, wall boards, reinforced concrete panels and the like, and pre-mixtures for forming the above formed building materials such as, for example, concrete mix, plaster, render and the like. Thus, in a preferred embodiment of the invention, the shielding material of the invention will take the form of a pre-mixture of the aforementioned shielding component (s) and building materials which will optionally include a hydraulically settable binder or cementitions material.

The invention also relates to the use of one or more of the aforementioned shielding components in the manufacture of building materials, or pre-mixtures, for building components .

The invention also relates to the use of one or more of the aforementioned shielding components in the manufacture of a building material for use in shielding an interior of a building from EM radiation, particularly EM radiation in the Hz, KHz, MHz or GHz range.

In another embodiment the invention provides a shielding material according to the -invention for use in reducing radar reflections from buildings and other structures such as, for example, airport runways. This is of particular importance with regard to both the protection of buildings of military importance and to airport buildings that can produce spurious radar reflections. Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is an illustration of a waveguide apparatus for measuring the shielding properties of a shielding material from 2.2 to 4.4 GHz according to the invention;

Fig. 2 is an illustration of a parallel plate apparatus for measuring the shielding properties of a shielding material in the kHz and MHz ranges according to the invention;

Fig. 3 is an illustration of the apparatus used in measuring the magnetic properties of a shielding material at 50 Hz according to the invention; and

Fig. 4 is a graphical representation of measurement data shown in table 3.

High Frequency Shielding Measurements

Wave Guide Measurements

Referring to Fig. 1, two wave guide to co-axial adapters 1 were mounted back to back, with the sample 2 on a mount 3 contained between waveguide flanges 4 with signal inputs/outputs 5. This arrangement holds the sample in a resonant cavity, with minimal signal leakage at the sample edges .

Plaques were scanned from 2.2 to 4.4GHz . Insertion loss (or transmission) and reflection were measured. Results at 3GHz are given in Table 1. Insertion loss is a measure of the output power from one face of the sample, compared with the input power on the opposite face, therefore a large negative value is preferable for a good shielding material, for example, sample 173 which gives an insertion loss of -30.49 dB at 3 GHz (Table 1). Insertion loss includes loss by absorption and reflection. Reflection values are the ratio or power reflected out from one face of the sample to the power input to the same face, therefore a small negative value represents good reflection, for example sample 172 which gives a reflection value of - 0.97 dB at 3 GHz (Table 1) .

An attenuation (or insertion loss) of -20 dB using the waveguide equipment means that if, for example, 1 Watt of power was incident on one face of the shielding material, then 0.01 Watts would be emitted from the opposite face.

Attenuation due to absorption was calculated as follows :

dB = 101og (P_out/P_in) Where P_out and Pi_n are the output and input power respectively.

For example, sample 170 (standard, no shielding additive) :

dB_refχ_ectΘd = -3.68

P_out/Pi_n = lO^"3-⁵⁸'¹⁰

= 0.423

Therefore, if P_in is 1 Watt, 0.423 Watts are reflected and 0.577 Watts are transmitted or absorbed.

dBi_nsθrtioιι loss ⁼ -2.96

Pout/Pin = 0.506

Therefore, 71 mWatts (7.1% of total incident power) is absorbed:

dB_absorbβ = 101og0.929

= -0.32 Sample preparation

Dry components were mechanically mixed for three minutes. Water was added and mixing continued for a further three minutes . Plaques were cast and hand tamped into wax moulds, based on compositions in Table 1.

Plaques of dimensions 10 x 74 x 136mm were cast.

Summary

^• Referring to Table 1, OPC/sand-based samples containing kish graphite, acetylene black or iron oxide give the best attenuation, with insertion losses up to -38dB (e.g. samples 172, 173, 185, 187 and 316) . For example, sample 172 (kish) reflects around 80% of the incident power. Substitution of 16 parts of kish with acetylene black slightly increases the insertion loss from -29.1dB to - 30.5dB. Reflection is reduced to 73% of incident power and absorption is increased.

Acetylene black is a form of carbon black. Sample 187 (75 parts acetylene black) gives an insertion loss of -30dB and a reflection of -1.5dB. This means that approximately 70.5% of incident power is reflected and 29% absorbed.

Plaster-based plaques 193, 194, 195 contain the same shielding component as PC sand-based samples 173, 180, 172. Insertion loss is greatest for the PC/sand formulations.

Broadly speaking, low resistivity samples and low resistivity shielding components give a high insertion loss, especially due to reflection.

The best absorbers tested to date are:

Samples 180 (60 parts lamp black 101) , which gives an insertion loss of -19.5dB and absorbs 49% of incident power.

Sample 277 (plaster, kish and magnetite) gives an insertion loss of -18.9dB and absorbs 44% of incident power.

Medium Frequency Shielding Measurements

Plate Line System

Referring to Fig. 2, a parallel plate line was set up following RAE Tech Memorandum FS(S) 510 and US MILSPEC 462. This consists of two aluminium plates mounted on plywood. The bottom plate 13 is earthed to a 0.75m² buried copper plate. All equipment is earthed to this plate. Resistors on the input coaxial plug are 0.25 Watt non-inductive, with short leads; 75 Ohm to earth and 60 Ohm feed to the line.

At the other end of the line, the top 12 and bottom 13 plates are connected by 3 x 270 Ohm 0.25 Watt non-inductive resistors, mounted on copper teeth, to form a 90 Ohm load. The input/output resistors are designed to match the size and separation of the two line plates to give a 50 Ohm signal impedance. Changes in the field distribution between the plates due to the sample are measured using a spectrum analyser and tracker generator (Hameg 5014) .

The sample 10 is mounted via foam strips, coated with aluminium foil, on an intermediate plate. The sample 10 is sited over a hole in the plate, 255 x 150mm. A 10mm band of silver paint is applied to the lower face of the sample to ensure electrical contact with the plate, which is earthed via brass legs 14 to the bottom plate 13.

The detector 15 is positioned below the centre of the sample, on the bottom plate. It consists of a 28cm² plate antenna and a bipolar MOSFET amplifier, powered by a 9v battery.

An attenuation of - 20 dB W using the plate line equipment means that if 1 Watt of power was incident on one face of the sample, then 0.01 Watts would be emitted from the opposite face.

Sample Preparation

Dry components were dry mixed for 3 minutes in a Hobart mixer. Water was added and mixing continued for a further 3 minutes. In formulations containing carbon black, the carbon was sheared into dry cement using a shear mixer.

The total mix was cast into a lined mould 275 x 175 x approximately 25mm, and compacted with a steel roller.

Measurements

Samples were scanned from 100 to 900kHz and from 5 to 55MHz. In addition, attenuation values in dBmW were recorded at 300kHz, 800kHz, 10MHz and 40MHz (see Table 2) . Empty measurements were made for each test session.

Results

Referring to Table 2, many formulations gave an attenuation of better than -20dBmW over the range 300kHz, e.g. samples 252 to 257. Shielding components included kish, lampblack 101, Silvershine graphite, chopped carbon fibre scrap and carborundum.

Wire mesh was included in tests because this is a currently acceptable shielding material, and is used as an alternative to steel cladding. In our tests wire mesh performed well in the kHz range but gave an attenuation worse than -20dBmW at MHz e.g. 259, 265, 266. Samples containing magnetic materials such as millscale or magnetite also gave good attenuation, especially at 300 to 800kHz, where many samples have better than -50dBmW, e.g. 284 (graphite and iron oxide) , 285 (lampblack and magnetite) and 276 (millscale and kish) .

Low Frequency Magnetic Shielding Measurements

The following tests were made to compare the magnetic shielding of plaques, each having a dimension of 275 x 175 x ~25mm. The manufacture of the plaques follows the same protocol as that described previously.

Referring to Fig. 3, a sample 20 was sandwiched between two U-cores 21, 22, with coil 24, mounted on the core. The coil 24 was wound to produce 1 Tesla at 240 Volts. 50 Volts rms was applied to the coil 34 and the input current measured.

The aim of the test is to provide a more efficient path for the magnetic circuit via the sample and so reduce the current input.

Materials with a high relative permeability (ie permeability relative to free space) (μ_r) are usually good magnetic shields. Materials which have a high relative permeability include :

Cast iron μ_r = 100 - 250 Mild steel μ_r = 200 - 800 Cast steel μ_r = 300 - 900 Mu-metal μ_r = 200 - 5000

Permeability varies with magnetic field strength; therefore, these values are given as ranges. The magnetic field strength (H) in a material can be expressed as :

H = NI amp.m^"1 L Where: N = Number of turns I = Current in amps L = Mean length of flux path in metres

And relative permeability is given by :

μ_r = B Hμ₀

= BL μ₀ΝI

Where: μo = permeability of free space B = magnetic flux density

Hence, a low input current (I) is proportional to a high permeability. Therefore, changes in the current through the input coil were measured. Results

Referring to Table 3 and Fig 4, the best coupling was achieved with samples containing swarf, steel mesh, millscale and magnetite. The current input for the standard formulation (250 no shielding component) was 0.318 Amps. Current input decreased with addition of shielding materials to 0.194 Amps (at 50 V input) for sample 296 (containing magnetite) .

Potential Shielding Components

Tables 4 to 6 provide, for each of the shielding components listed, suggested loading amounts per frequency range. Table 7 gives volume resistivities of bulk powdered components .

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail without departing from the scope of the invention.

l . Shieldin D ta at GHZ

CO c

CD CO

m co m m

c m r

hieldin R sults 300kHz to 3 GHz

CO

C CO CO

m co

I D m m

H c m ro

CO c

CO CO

m co t m m

73 c m r

CO

C CO CO

m co m m r t c m r

Table 3. Imput Current at 50 V Input for Ferrous Components

CO c

CO CO

m co t m m

c m r

Table 4. Shieldin Com onents GHz Ran e

Table 5. Shieldinq Components, kHz and MHz Ranqes

Table 6. Components for Shielding at 50Hz

Table 7. Volume Resistivitv of Bulk Particulate Components

Claims

1. A shielding material comprising at least one basic building material, optionally together with a hydraulically settable binder or cementicious material, and at least one shielding component capable of reflecting or absorbing electromagnetic radiation (EM) .

2. A shielding material according to claim 1 in which the hydraulic binder is any material which when mixed with water sets and hardens to form a solid mass either at ambient conditions of temperature and humidity or at elevated temperatures and saturated conditions.

3. A shielding material according to Claim 2 in which the hydraulic binder is selected from the group comprising: Portland cement, calcium aluminate cement, calcium sufoaluminate cement, calcium sulfate hemihydrate (Plaster of Paris) and calcium sulfate anhydrite.

4. A shielding material according to Claim 2 or Claim 3 in which the hydraulic binder comprises Portland cement in combination with one or more of the following materials : Pozzolanic materials such as pulverised fuel ash (PFA) or volcanic ash etc.; latent hydraulic materials such as ground granulated blastfurnace slag (GGBS) and other slags; finely ground silica; microsilica; metakaolin; lime; ground limestone or other mixtures thereof. 5. A shielding material as claimed in any preceding claim in which at least one shielding component having reflective properties is chosen from the group consisting of; kish graphite, coke, exfoliated graphite, exfoliating graphite, artificial graphite (for example electrode graphite), graphitised carbon fibres (pitch or polyacrylonitrile based, inc. scrap fibres), natural graphite, flake graphite, millscale, steel fibre, steel swarf, non-graphitised carbon fibre, oxidised carbon fibre, a carbon black such as acetylene black, conductive black, or rubber black and PFA.

6. A shielding material as claimed in any preceding claim in which at least one shielding component having absorptive properties is chosen from the group consisting of: a carbon black such as lamp black, channel black, thermal black, furnace black or rubber black, low rank coal charcoal, viscose based carbon fibre, carborundum, ferrite, and PFA.

7. A shielding material as claimed in any preceding claim in which the shielding component (s) capable of reflecting or absorbing radiation comprises a ferrous material.

8. A shielding material as claimed in claim 7 in which the ferrous material comprises magnetite, millscale or an iron oxide. 9. A shielding material as claimed in any of claims 1 to 8 in which the component (s) capable of reflecting or absorbing radiation comprise graphite and/or a carbon black.

10. A shielding material as claimed in Claim 9 in which the graphite is kish graphite.

11. A shielding material as claimed in Claim 9 or Claim 10 in which the carbon black is acetylene black.

12. A shielding material as claimed in any of Claims 9 to 11 in which the graphite comprises between 0.5 and 60% of the shielding material (w/w dry components) .

13. A shielding material as claimed in Claim 12 in which the graphite comprises between 10 and 40% of the shielding material (w/w dry components) .

14. A shielding material as claimed in any of Claims 9 to 13 in which the carbon black comprises at least 0.5% of the shielding material (w/w dry components) .

15. A shielding material as claimed in claim 14 in which the carbon black comprises between 1 and 20% of the shielding material (w/w dry components) . 16. A shielding material as claimed in claim 15 in which the carbon black comprises between 3 and 10% of the shielding material (w/w dry components) .

17. A shielding material as claimed in Claim 16 in which the carbon black comprises between 4 and 6% of the shielding material (w/w dry components) .

18. A shielding material as claimed in Claim 9 in which the shielding component (s) capable of reflecting or absorbing radiation comprises a combination of graphite, especially kish graphite, and carbon black, especially acetylene black, and optionally magnetite.

19. A shielding material as claimed in Claim 18 in which the graphite comprise between 0.5 and 50% of the shielding material (w/w dry components) .

20. A shielding material as claimed in Claim 19 in which the graphite comprise between 20 and 40%, of the shielding material (w/w dry components) .

21. A shielding material as claimed in any of Claims 18 to 20 in which the carbon black comprises between 0.5 and 10% of the shielding material (w/w dry components) .

22. A shielding material as claimed in Claim 21 in which the carbon black comprises between 1 and 5% of the shielding material (w/w dry components) . 23. A building component comprising a shielding material according to any of Claims 1 to 21.

24. Use of a shielding material according to any of Claims 1 to 22 in the manufacture of dwelling houses and apartments and industrial buildings.

25. Use of a shielding material according to any of Claims 1 to 22 in reducing radar reflections from buildings.

26. Use of kish graphite in the manufacture of building materials for buildings for shielding an interior of the building from EM radiation in the Hz to GHz range .

27. Use of ferrous material such as magnetite or an iron oxide in the manufacture of building materials for buildings for shielding an interior of the building from EM radiation in the Hz to GHz range.

28. Use as claimed in Claim 26 or Claim 27 in which the kish graphite and the ferrous material is provided in the form of a powdered or particulate material.