CA1330704C

CA1330704C - Electromagnetic wave absorbing material

Info

Publication number: CA1330704C
Application number: CA 576414
Authority: CA
Inventors: Takemi Yamamura; Toshihiro Ishikawa; Masaki Shibuya
Original assignee: Ube Industries Ltd
Current assignee: Ube Corp
Priority date: 1987-09-04
Filing date: 1988-09-02
Publication date: 1994-07-19
Anticipated expiration: 2011-07-19
Also published as: DE3889042D1; JPH071837B2; JPH01157598A; DE3889042T2; EP0306311B1; US5094907A; EP0306311A1

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is an electromagnetic wave absorbing structure comprising:
[A] a composite of a plurality of laminated layers of an electromagnetic wave absorbing material comprising a fiber having a specific resistance of 10-2 to 102 .OMEGA..cm and a matrix, wherein the fiber is composed of an inorganic substance selected from the group consisting of:
i) an amorphous substance substantially composed of Si, M, C and O, ii) crystalline ultrafine particles substantially composed of .beta.-SiC, C, MC, a solid solution of .beta.-SiC and MC and/or MC1-x, and having a particle diameter of not more than 500 .ANG., or alternatively, an aggregate of the crystalline ultrafine particles, amorphous SiO2 and amorphous MO2, and iii) a mixture of the above amorphous substance i) with the above crystalline ultrafine particles or aggregate ii) in which M denotes Ti or Zr, and x is a member more than 0 but less than 1, and [B] an electromagnetic wave transmitting material laminated on a surface of the composite [A].

Description

1 3:~070~

Field of the Invention This invention relates to an electromagnetic wave absorbing structural material having high strength and elastic modulus and excellent heat resistance.
Descripkion of the Prlor Ar~
Electroma~netic wave absorb1ng materials are used to prevent the microwave leakage in openings of microwave-heat cookers, electrowave darkrooms, etc. Further, electromagnetlc wave absorbing materials are used as a radar wave absorbing material to - ;
prevent a ship, airplane, etc., from being detected by a radar.
Several electromagnetic wave absorbing material~ have been proposed. ;~
Japanese Patent Publication No. 31275/1978 discloses an electromagnetic wave absorbing material formed by inaorporating .~ . .
carbon black and fine metal particles into a resin. This ~ ~
electromagnetic wave absorbing material has, however, low strength ~ ;
and elastic modulus, and therefore can not be used as a structural material.
Japanese Laid-Open Patent Publication No. 66699tl982 ,~, . .
discloses an electromagnetic wave absorbing material made of a aomposite material consistlng of a carbon 1ber having a complex ~;
speclflc dielectric constant'of [~- ~8 - 12) -~(3 - 5)] at 10 GHz o~ fre~uencies and a resin.
Since the carbon ~iber exhibits metallic conduction, its specific resistance increases as the temperature rises. Hence, its electromagnetic wave absorption property lower to a great extent.

~ ~'.:

:-~ 1 330704 The electromagnetic wave absorbing material is required to have an unchanged electromagnetic wave absorption property over a wide temperature range. From this viewpolnt, the electromagnetic wave absorbing material descrlbed in the above Japanese Laid-Open Patent Publication is not practically satisfactory.
European Laid-Open Patent Publication 206536 describes an inorganic fiber-reinforced plastic composite material composed of plastics and an amorphous or microcrystal inorganic fiber consisting of silicon, carbon, titanium or zirconium and oxygen.
However, said European Laid-Open Patent Publication describes nothing concerning the specific resistance of the lnorganic fiber used, nor does it describe anything about the fact that a composi~e material composed of an inoryanic fiber having a particular specific resistanae and plastlcs can be used as an electromagnetic wave absorbing material.
Summary of the Invention According to this invention, there is provided an -electromagne~ic wave absorbing structural material comprisings lA] a composite of a plurality of laminated layers of an electromagnetic wave absorbing material composed of a composite material comprising a fiber having a specific reslstance of 10 to 102 Q.cm and a matrix,Iwherein the fiber is composed of an inorganic substance selected from the group consisting of~
li) an amorphous substance substantially composed of Si, M, C and 0, (ii) crystalline ultrafine partlcles substantially composed of ~-SiC, C, MC and at least one member ~elected from the ~ :" 1330704 group conslsting of (a) MCl x and (b) a solid solution of B-SiC
and MC, and havlnq a parti.cle dlame~er of not more than 500 R, or an aggregate of the crystalllne ultrafine partlcles, amorphous .
SiO2 and amorphous M02, and tiil) a mlxture of the above amorphous substance i) wlth the above crystalline ultrafine particles or aggregate il), in which M is Ti or Zr, and x ls a number of more than 0 but less than 1, and [B] an electromagnetic wave transmitting material laminated ~ .
on a surface of the composlte lAl- - :
Dstailed Description of the Invention The electromagnetic wave absorblng material is composed ~:
of a composlte material comprlslng a fiber havlng a speclfic resistance of 10 2 to 102 Q.cm and a matrix and effectively -~
absorbing microwaves ln ~he range of from 500 MHz to 3,000 GHz.
The fiber usable in the electromagnetic wave absorblng ~aterial can be prepared, for example, accordin~ to a method descrlbed in U.S. Patent~ 4,342,712 and 4,315,742. .
The above flber can be prepared by, flrgt, reactlng ::~
polycarbosllane composed mainly of (Sl-CH~) bond units and havlng a number average molecular weight of 200 to 10,000 and tltanlum alkoxlde or zlrconlum alkoxlde in an lnert atmosphere under heat :~
to bond at least a part of the slllcon atoms of the ~ : ;
polycarbosiIane with tltanium or zlrconlum of the above alkoxide~ .
~ through an oxygen atom wherehy polytltanocarbosllane or ~.~
; polyzlrconocarbosllane having a number average molecular welght of ~::
: 1,000 to 50,000 is formed, then preparlng a spinnlng solutlon of ~.

A - 3 ~ ~.
., ' ~

1 33070~ 72860-2 the polytitanocarbosilane or polyzlrconocarbosllane, spinning a fiber from the solution, rendering the fiber lnfusible, and flnally subjecting the lnfuslble fiber to heat treatment under vacuum, inert ga~ or reducing gas atmosphere at 1,300 ~o 1,500 C. As the temperature for the above heat treatment increases within the above temperature range, the specific resistance of ~he resultant fiber decreases.
The fiber usable in the electromagnetic wave absorblng ~
material can also be prepared by other methods described in U.S. ~ -Patents 4,342,712 and 4,315,742.
That is, the method described in these patents comprises :~
heating polycarbosilane having a number average molecular weight of 500 to 1,000 and polytitanocarbosilane or polyzirconocarbosilane having a number average molecular weight of ~ :
500 to 1,000 in an organic solvent under lnert gas atmosphere, to bond at least a part of the silicon atoms of the polycarbosilane ::-with silicon atoms, titanium atoms or zirconium atoms of the ~ ~
polytitanocarbosilane or polyzirconocarbosilane through an oxygen ~ -atom, whereby an organic silicon polymer is prepared, then preparing a spinning solution of this polymer, spinning a fiber from the solution rendering the spun fiber infusible, and finally . . . :, , ., ~
~ub~ecting the infusible fib!er to heat treatment under vacuum, inert gas or reducing gas atmosphere at 1,300 to l,S00 C.
The proportions of the elements in the fiber are, in ~: ;
general, as follows.

:.
- 4 - ~;

1 3;~0704 ~i : 28 to 60~ by weight C : 23 to 60% by weight Ti or Zr : 0.5 to 30% by weight o ~ 1 to 30% by weight No ~pecial limitatlon is imposed on the form of the fiber. However, a continuo~s fiber having a diameter of 5 to lS ~m is pre~erable in order to obtain an electromagnetic wave absorbing material having good mechanical property and electromagnetic wave absorbing property.
Examples of the fibers are those which are monoaxially or multiaxially oriented, or ~hich are various fabrics such as plain weave, æatin elastic webbing, twilled weave, leno cloth, spiral weave, three dimensional weave, etc.
The matrix for use in the electromagnetic wave absorbing material may be of plastics or ceramics.
~ xamples of the plas~ic matrix includes epoxy resin, polyurethane resin, polyamide resin, polycaxbonate resin, silicon ;~
resiin, phenoxy resin, polyphenylene sulflde resin, fluorine resin, .: :: ..
hydrocarbon resin, halogen-containing resln, acryllc acid-t~pe resin, acryloni~rlle~butadiene/styrene resin, and ultrahigh molecular weight polyethylene.
Among the abovejplastlc matrices, epoxy resin lsi preferably used. The epoxy resln is a resin composition composed of polyepoxide, curlng agent, auring catalyst, etc.
Examples of the polyepoxide lncludes a glycidyl compound of bisphenols A, F and S, a ylycldyl compound of cresol novolak or phenol novolak, al~cyclic polyepoxide, etc. The other examples of ~:;

1 33070~

the polyepoxide include a glycidyl compound of polyphenol, polyhydric alcohol or aromatic amine.
Among these polyepoxides, generally used are a glycidyl compound of bisphenol A, a glycidyl compound of diiminodiphenyl methane and a glycidyl compound of aminophenol. When the electromagnetic wave absorbing material of this invention is used as a mPmber material requiring high functions such as a primary structural material of airplanes, it is preferable to use glycidyl compounds of polyfunctional amines such a~ diiminodiphenyl methane.
Examples of the ceramic matrix include carbide ceramics such as silicon carbide, titanium carbide, zirconium carbide, nioblum carbide, tantalum carbide, boron carbide, chromlum carbide, tun~sten carbide, molybdenum carbide, etc.: nitride aeramlcs such as silicon nitride, titanium nitride, zirconium . .:, ;, , ~ ~, nitride, vanadium nitride, niobium nitride, tantalum nitride, boron nitride, hafnium nitride, etc.; oxide ceramics such as alumina, magnesla, mullite, cordierite, etc.
In the case when the matrix is of plastic, the composite material may be prepared by the hand lay up method, matched metal die method, break awaY method, fllament winding method, hot press method, autobreak method, continuous drawing method, etc.
When the matrix is of ceramic, there is a method of preparing an aggregate of a fiber and ceramic powder and sintering the aggregate. For the preparation of the above aggregate, there can be employed those methods of embedding a ceramlc powder alternately, filling a ceramic powder in the spaces formed among 1 33U70~

prearranqed fibers, etc.
The electromagnetic wave absorbing material of this invention has a structure formed, in general, by arranging the above fibers of various forms in the matrix to form compo~ites and laminating a plurality of the resultant composites integrally.
Especially, preferable is a structure formed by the method which comprises laminatiny a plurality of composites which are prepared by arranging fibers monoaxially. Examples of the lamination method inaludes those of laminating composites such that the directions of the fibers are in agreement, laminatlng composites such that the fiber direction of one composite is at right angles to the fiber direction of another composite, alternately, etc. The latter method is preferable to obtain good electromagnetic wave absorbing -~property.
The electroma~netic wave absorbing materlal has a thickness of, in qeneral, 1 to 10 mm. The proportion of the fiber ;~
in the electromagnetic wave absorbing material is, preferably, 30 to 80% by volume, especially preferably 45 to 65% by volume.
The electromagnetic wave absorbing material is laminated wlth an electromagnetic wave transmitting material on a surface.
It is well known that when an electromagnetic wave contacts a material~which has~an extremely larger dielectric ~
constant than the air, reflectlon of the electromagnetic wave on ~ ;
the surface layer increases. In the electromagnetic wave absorbing material, lt ls required that the amount of reflection on the surface be very small. Further, if the laminatlon structurq is so formed as to maka the dielectric aonstants gradually larger from the surface inward, the above reflection of the electromagnetic wave on the surface layer decreases to a great extent.
Examples of the electromagnetic wave transmlttin~
material for use in aombination wi~h the electromagnetic wave absorbing material include gla~s fiber- or aromatia polyamide fiber- reinforced polyester resins.
Preferably used electromagnetic wave transmitting material is a aomposite materlal which compri~es a ~iber composed of a substantially amorphous substance made of sllicon, carbon titanium or zirconium and oxygen and having a specific resistance ~-of 105 to 101 Q.cm and a matrix.
The above fiber may be prepared in the same way as the fiber for the electromagnetic wave absorbing material except that the infusible ~iber is subjected to heat treatment at 800 to 1,300 C and that the time period for the heating is suitably seleated.
The proportions of the above elements for use in the eleatromagnetia wave transmltting material are, in general, as followsl -, Si . 26 to 58% by weight C , 25 to 62% by weight ;
Ti or Zr 0.5 to 30~ by weight and 0 ~ 1 to 30% by welght. ~;
The ~atrix and the aomposite of the fiber and the matrix are prepared in the same way as those for the electromagnetic wave absorbing material.

- 7a -~A ~` ~

1 33070~

The electromagnetlc wave transmitting material layer laminated on ~he electromagnetic wave absorbing material usually has a thickness of 0.1 to 5 mm. And the proportion of the fiber in the electromagnetic wave transmitting material is preferably, 30 to 80% by volume, e~pecially 45 to 65% by volume.
The electromagnetic wave absorbing materlal can absorb microwaves of 500 MHz to 3,000 GHz, particularly mlcrowave~ of 2 IO 20 GHz. The fiber in the electromagnetic wave abeorbiny material of this invention has semiconductor type property, very ~mall activation energy, 0.01 eV, of electric conductivity and no change in the electromagnetic wave absorbing characteristlc even with temperature changes. Therefore, the electromagnetic wave absorbing material o~ this invention exhlbits excellent electromagnetic wave absorbing performance over a wide temperature range. Further, the electromagnetic wave absorbing materlal of this invention has high strength and elastlc modulus and excellent heat resistance, and therefore, can be u~ed as a primar~
structural materlal ~or shlps, alrplanes, etc.
Examples The ~ollowlng Examples lllustrate this lnventlon. In the Examples, the re~lection attenuation amount of an electromagnetlc wave in an eleatrqmagnetic wave absorbing material ~as measured with regard to a 22.86 mm x 10.16 mm x 2.00 mm sample by the use oi an S-parameter measuring apparatus made by Yokogawa-Hewlett- -~
Packard, Ltd. The apparatus "

~ ;
- 7b -1 33070~

consists of an S-parameter measuring device HP~515A, network analayzer HP8510 and synthesized sweeper HP8340A. The reflection attenuation amount (db) of an electromagnetic wave absorbing material was measured at 25 ~.

2.5 ~ of anhydrous xylene and 400 g of sodium were heated under a nitrogen gas current up to the boiling point of xylene, and 1 Q of dimethylchlorosilane was added dropwise over 1 hour. After the addition, the mixture was refluxed under heat for 10 hours to form a precipitate. The precipitate was filtered, and washed with methanol and then with water to give 420 g of a white powder of polydimethylsilane.
The above polydimethylsilane was charged into a flask having a gas introducing tube, stirrer, condenser and distillation outlet tube, and treated under heat at 420 ~
under nitrogen gas current with stirring, to give 350 g of a colorless transparent and a little viscous liquid in a distillate receptor. The li~uid had a number average molecular weight, measured by vapor pressure infiltration method, of 470. IR spectrum measurement showed that this substance was an organic silicon polymer having a total number of (Si-CH2) bond units/ total number of (Si-Si) bond units ratio of 1 : 3. -~

3 g of polyborodiphçnylsiloxane was added to 100 g of the above organic silicon polymer, and the mixture was thermally condensed at 350 ~ to give a polycarbosilane having ~ main chain skeletodn mainly composed of carbosilane units of :A ~ormula (Si-CH2) ~ having a hydrogen atom and methyl group attached to the silicon atom of the carbosilane unit.
Tetrabutoxysilane was added to the resultant polycarbosilane, and'the mixture was crosslinkage-polymerized in nitrogen atmosphere at 340 ~ to give a polytitanocarbo-silane composed of 100 parts of (Si-Si) units and 10 parts of (Ti-O) units. This polymer was melt-spun, and the spun fiber was treated in the air at 190 ~ so as to render it infusible, and then fired in nitrogen atmosphere at 1,500 ~.
The resultant fiber (fiber~I]) had a diameter o~ 10 ~m, tensile strength of 300 kg/mm2, tensile elastic modulus of . .

~ t 330704 16 t/mm2 and specific resistance of 1.5 Q-cm.

The procedure of Referential Example 1 was repeated except that a fiber which was rendered infusible in Referential Example 1 was fired at 1,200 ~, and as a result,a fiber [~] was obtained.
The resultant fiber [D ] had a diameter of 10 ~m, tensile strength of 280 kg/mm2, tensile elastic modulus of 15.5 t/mm2 and specific resistance of 2 x 105 Q cm.

The procedure of Referential Example 1 was repeated except that a fiber which was rendered infusible in Referential Example 1 was fired at 1,050 ~, and as a result,a fiber ~] was obtained.
The resultant fiber [~] had a diameter of 10.5 ~m, tensile strength of 260 kg/mm2, tensile elastic modulus of 15 t/mm2 and specific resistance of 3 x 107 Q-cm.
~ EFERENTIAL EXAMPLE 4 The procedure of Referential Example 1 was repeated except that a fiber which was rendered infusible in Re~erential Example 1 was fired at 1,400 ~, and as a result,a fiber l~] was obtained.
The resultant fiber [~] had a diameter of 10 ~m, tensile strength of 305 kg/mm2, tensile elastic modulus of 16.5 t/mm~ and specific resistance of 40 Q cm.

Zirconiumtetrabutoxide was added to a polycarbosilane prepared in the same way as in Referential Example 1, and the mixture was crosslinkage-polymerized in nitrogen atmosphere atl250 ~ to give a polyzirconocarbosilane composed of 100 parts of (Si-CH2) units and 10 parts of (Zr-O~
units. This polymer was melt-spun, and the spun fiber was treated in the air at 190 ~ so as to render it infusible and then fired in nitrogen atmosphere at 1,500 ~.
The resultant ~iber (fiber [~']) had a diameter of 10 ~m, tensile strength of 310 kg/mm2, tensile elastic modulus of 16.5 t/mm2 and specific resistance of 2.0 Q-cm.
' _9,_ 100 parts by weight of bisphenol A-type epoxy resin (XB 2879A made by Ciba Geigy) and 20 parts by weight of dicyanamide curing agent ~XB 2879B made by Ciba Geigy) were uniformly mixed with each other, and then the mixture was dissolved in a methylcellulose/acetone (1:1 by weight) mixed solvent to prepare a solution of 28 -~ by weight of the above mixture in the above mixed solvent.
The ~iber [I] was immersed in the ~bove solution, then taken up monodirectionally by using a drum winder and heated in a heated air-circulating oven at 100 ~ for 14 minutes to prepare a prepreg in the semi-cured state. The prepreg had a resin content of 50 ~ by volume and thickness of 0.2 mm.
11 sheets of the prepreg were laminated such that the direction of the fiber of one sheet was at right angles to the direction of the fiber of another sheet adjacent thereto, and the laminate was press-shaped at 130 ~ by a press pressure of 11 kg/cm2 for 90 minutes to give a composite A having a size of 250 mm x 250 mm x 2 mm.
Separately, the above procedure was repeated except~
the use o~ the fiber [~] to prepare a prepreg having a resin content of 5~ % by volume and thickness of 0.2 mm. Then, 16 sheets of said prepreg were used and the above procedure was repeated to give a composite B haviny a size of 250 mm x 250 mm x 2 mm.
A composite A' and composite B' each having a size of 22.86 mm x 10.16 mm x 2 mm were taken from the above composite A and B by cutting them off, and these two composites were adhered to each other by an epoxy solutioh -~
used in preparation of the prepreg such that the directions of the fibers were arranged in the same direction, to prepare an electromagnetic wave absorbing material.
When a microwave was introduced into the above 61,b ~o r b ~ 9 electromagnetic wave ~Js~ y material from the side of the composite containing the fiber ~l], ~ material exhibited a reflection attenuation amount of not less than 15 db j! ... . , '.' '.' ' 'i .' ... ',,: ' .,; . ','.' '.. ' '.' ', ' :: . i .

(reflectance of not more than 3.2%) ln the frequency region of 5 to 20 GHz. The materlal, in a slmilar measurement at 150C, showed reflection attenuation amounts which were about the same as the above value.
The above electromagne~ic wave absorbing material had a flexural strength of 102 kg/mm2 and tenslle strenyth of 95 kg/mm2 when measured in the flber dlrectlon.

Example 1 was repeated except that the fiber [III] was used in place of the fiber [TI], to prepare a composite C.
Then, an electromagnetic wave absiorbing material was prepared from the above composite C and the composite A obtained in Example 1 in the same way as in Example 1.
When a microwave was lntroduced into the above electromagnetia wave absorbing material from the side of the ~ -composite containing the fiber lIIIl, the m~terial e~hibited a re~lection attenuatlon amount of 40 db at 10 GHz.
The above electromagnetlc wave absorblng material had a flexural strength of 108 kg/mm2 and tensile strength of ~7 kg/mm when measured ln the flber direction.

Example 1 was repeated except that the fiber lIV~ was used in place of the fiber [I], to prepare a prepreg having a resin content of 50~ by volume and thickness of 0.2 mm. The procedure of Exa~ple 1 was repeated except that 16 sheets of the ~ ;

) . ;~ -- 11 -~ ' : :

above prepreg were used to obtain an electromagnetlc wave absorbing material composed of a composite D havlng a size 250 mm x 250 mm x 3 mm.
When a microwave was introduced into the above electromagnetlc wave absorbing material, the material exhibited a reflection attenuation amount of 30 db at 10 GHz.
The above electromagnetic wave absorbing material had a flexural strength of 105 kg/mm~ and tensile strength of 96 kg/mm2 when measured in the fiber direction.

Example 1 was repeated except that the fiber lV] was used in place of the fiber [I~, ~o prepare an electromagnetic - lla -wave absorbing material.
When a microwave was introduced into the above electromagnetic wave absobing materia~ from the side of the composite containing the fiber [~], ~ material exhibited a :;
reflection attenuation amount of not less than 14 db in the fregiuency region of 5 to 20 GHz.
The above electromagnetic wave absorbing material had a flexural strength of 105 kg/mmZ and tensile strength of 96 kg/mm2 when measured in the fiber direction.

'

Claims

1. An electromagnetic wave absorbing structural material comprising:
[A] a composite of a plurality of laminated layers of an electromagnetic wave absorbing material composed of a composite material comprising a fiber having a specific resistance of 10-2 to 102 .OMEGA..cm and a matrix, wherein the fiber is composed of an inorganic substance selected from the group consisting of:
(i) an amorphous substance substantially composed of Si, M, C and O, (ii) crystalline ultrafine particles substantially composed of .beta.-SiC, C, MC and at least one member selected from the group consisting of (a) MC1-x and (b) a solid solution of .beta.-SiC
and MC, and having a particle diameter of not more than 500 .ANG., or an aggregate of the crystalline ultrafine particles, amorphous SiO2 and amorphous MO2, and (iii) a mixture of the above amorphous substance i) with the above crystalline ultrafine particles or aggregate ii), in which M is Ti or Zr, and x is a number of more than 0 but less than 1, and [B] an electromagnetic wave transmitting material laminated on a surface of the composite [A].

2. A structural material according to claim 1, wherein the composite material in the electromagnetic wave absorbing material absorbs electromagnetic waves in the range of from 500 MHz to

3,000 GHz.

3. A structural material according to claim 1, wherein in the electromagnetic wave adsorbing material the fiber is one obtained by carrying out the spinning of polytitanocarbosilane or polyzirconocarbosilane having a number average molecular weight of 1,000 to 50,000, rendering the spun fiber infusible and heating the infusible fiber under vacuum, inert gas or reducing gas atmosphere at 1,300 to 1,500°C.

4. A structural material according to claim 1, wherein in the electromagnetic wave absorbing material the fiber is one obtained by carrying out the spinning of an organic silicon polymer obtained by heating polycarbosilane having a number average molecular weight of 500 to 1,000 and polytitanocarbosilane or polyzirconocarbosilane having a number average molecular weight of 500 to 1,000 in an organic solvent under inert gas atmosphere, treating the spun fiber so as to render it infusible and heating the infusible fiber under vacuum, inert gas or reducing gas at 1,300 to 1,500°C.

5, A structural material according to claim 1, wherein in the electromagnetic wave absorbing material the proportions of elements in the fiber are as follows:
Si : 28 to 60 % by weight C : 23 to 60 % by weight Ti or Zr: 0.5 to 30% by weight O : 1 to 30 % by weight,

6. A structural material according to claim 1, wherein in the electromagnetic wave absorbing material the fiber is a continuous inorganic fiber having a diameter of 5 - 15 µm.

7. A structural material according to claim 1, wherein in the electromagnetic wave absorbing material the matrix is of plastic or ceramic.

8. A structural material according to claim 1, wherein in the electromagnetic wave absorbing material the proportion of the fiber in the composite material is 30 to 80% by volume.

9. A structural material according to claim 1, wherein the electromagnetic wave transmitting material is a glass fiber-or aromatic polyamide fiber-reinforced polyester resin.

10. A structural material according to claim 1, wherein the electromagnetic wave transmitting material is composed of a substantially amorphous fiber made of silicon, carbon, titanium or zirconium and oxygen and having a specific resistance of 105 to 1010 .OMEGA..cm and a matrix.

11. A structural material according to claim 10, wherein the substantially amorphous fiber is one obtained by carrying out the spinning of polytitanocarbosilane or polyzirconocarbosilane having a number average molecular weight of 1,000 to 50,000, rendering the spun fiber infusible and heating the infusible fiber under vacuum, inert gas or reducing gas atmosphere at 800 to 1,300°C.

12. A structural material according to claim 10, wherein the substantially amorphous fiber is one obtained by carrying out the spinning of an organic silicone polymer obtained by heating polycarbosilane having a number average molecular weight of 500 to 1,000 and polytitanocarbosilane or polyzirconocarbosilane having a number average molecular weight of 500 to 1,000- in an organic solvent under inert gas atmosphere, treating the spun fiber so as to render it infusible and heating the infusible fiber under vacuum, inert gas or reducing gas atmosphere at 800 to 1,300°C.

13. A structural material according to claim 10, wherein the proportions of elements in the substantially amorphous fiber are as follows:
Si : 28 to 58 % by weight C : 25 to 62 % by weight Ti or Zr: 0.5 to 30% by weight O : 1 to 30 % by weight .

14. A structural material according to claim 10, wherein the proportion of the fiber in the electromagnetic wave transmitting material is 30 to 80% by volume.

15. A structural material according to claim 2, wherein, the proportions of elements in the fiber of the electromagnetic wave absorbing material are as follows.
Si : 28 to 60% by weight, C : 23 to 60% by weight, Ti or Zr : 0.5 to 30% by weight, and O : 1 to 30% by weight;
the electromagnetic wave absorbing composite material contains 30 to 80% by volume of the fiber and the rest of the matrix which is a ceramic or plastic; and the electromagnetic wave transmitting material is a composite composed of a plurality of layers of a material comprising (a) 30 to 80% by volume of a substantially amorphous fiber made of:
Si : 26 to 58% by weight, C : 25 to 62% by weight, Ti or Zr : 0.5 to 30% by weight, and O : 1 to 30% by weight;
and having a specific resistance of 105 to 1010 .OMEGA..cm and (b) a matrix which is a ceramic or plastic.

16. A structural material according to claim 15, wherein the matrices of the electromagnetic wave absorbing material and the electromagnetic wave transmitting material are each a plastic.