Method for preparing composite materials with an oxide matrix and oxide reinforcements by means of a calefaction process

Abstract

Method for the preparation, by means of a heating technique, of a composite material composed of a matrix of at least a first oxide of at least one metal and/or at least one metalloid reinforced by reinforcements in at least a second oxide of at least one metal and/or at least one metalloid, characterised in that the following successive steps are carried out: the reinforcements are placed in at least one liquid precursor of the first oxide of at least one metal and/or at least one metalloid; said reinforcements and the liquid precursor are heated so as to form the first oxide by means of the thermal decomposition of said liquid precursor, and to deposit the first oxide thus formed around the reinforcements and between the reinforcements thus forming the matrix.

Claims

1. A method for preparing, by a film-boiling chemical vapour infiltration (CVI) technique, a composite material consisting of a matrix of at least one first oxide of at least one metal and/or at least one metalloid reinforced by reinforcements comprising at least one second oxide of at least one metal and/or at least one metalloid, the method consisting of, in the following order: disposing the reinforcements in at least one liquid precursor compound of the first oxide of at least one metal and/or at least one metalloid; and heating said reinforcements and the liquid precursor compound, forming a vapour barrier of the liquid precursor compound on the surface of the reinforcements, so as to form the first oxide by thermal decomposition of said liquid precursor compound, and to deposit the first oxide thus formed around the reinforcements and between the reinforcements thus forming the matrix.

2. The method according to claim 1, wherein the first oxide of at least one metal and/or at least one metalloid is selected from the group consisting of SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2, mullite, aluminosilicates, and mixtures thereof.

3. The method according to claim 1, wherein the second oxide of at least one metal and/or at least one metalloid is selected from the group consisting of silica, alumina, mullite, and mixtures thereof.

4. The method according to claim 1, wherein the reinforcements are fibres.

5. The method according to claim 4, wherein the fibres form threads, fabrics, felts or three-dimensional structures with fibres with a length dimension.

6. The method according to claim 1, wherein the liquid precursor compound has a boiling temperature of less than 300° C.

7. The method according to claim 1, wherein the liquid precursor compound is chosen from all the liquid precursor compounds used in the chemical vapour deposition (CVD) technique or in the sol-gel technique.

8. The method according to claim 1, wherein the liquid precursor compound is chosen from the organometallic compounds and the organometalloid compounds.

9. The method according to claim 8, wherein the organometallic compounds are chosen from the alkoxides/alcoholates of metals, and the organometalloid compounds are chosen from the alkoxides/alcoholates of metalloids.

10. The method according to claim 8, wherein the organometallic compounds are chosen from the organometallic compounds comprising at least one metal selected from the group consisting of zirconium, aluminium, titanium, cerium, yttrium, lanthanum, lead, tin, antimony, boron, vanadium, indium, niobium, bismuth and hafnium.

11. The method according to claim 8, wherein the organometallic compounds are at least one selected from the group consisting of trialkoxy aluminiums; aluminium acetylacetonate; tetra alkoxy zirconiums; and tetra alkoxy titaniums.

12. The method according to claim 8, wherein the organometalloid compounds are chosen from organosilanes.

13. The method according to claim 12, wherein the organometalloid compounds are selected from the group consisting of tetra alkoxy silanes, methyltrichlorosilane (MTS), dimethyldichlorosilane, and methyl dichlorosilane.

14. The method according to claim 1, wherein the reinforcements and the liquid precursor compound are heated at a temperature of 700° C. to 120° C., for a period of 5 to 120 minutes.

15. The method according to claim 1, wherein the reinforcements and the liquid precursor compound are heated by inductive heating or resistive heating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic vertical cross-sectional view of a calefaction densification reactor, known as “Kalamazoo” reactor that may be used to implement the method according to the invention, and that is used in the examples.

(2) FIG. 2 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E01, prepared in Example 1.

(3) The scale shown in FIG. 2 represents 100 μm.

(4) FIG. 3 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E02, prepared in Example 2.

(5) The scale shown in FIG. 3 represents 300 μm.

(6) FIG. 4 is a photograph taken in optical microscopy, at a higher magnification, of a cross-section, perpendicular to the carbon graphite support, of the sample E02, prepared in Example 2.

(7) The scale shown in FIG. 4 represents 100 μm.

(8) FIG. 5 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E03, prepared in Example 3.

(9) The scale shown in FIG. 5 represents 30 μm.

(10) FIG. 6 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E04, prepared in Example 4.

(11) The scale shown in FIG. 6 represents 50 μm.

(12) FIG. 7 is a photograph taken in optical microscopy, at a higher magnification, of a cross-section, perpendicular to the carbon graphite support, of the sample E04, prepared in Example 4.

(13) The scale shown in FIG. 7 represents 30 μm.

(14) FIG. 8 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E05, prepared in Example 5.

(15) The scale shown in FIG. 8 represents 300 μm.

(16) FIG. 9 is a photograph taken in optical microscopy, at a higher magnification, of a cross-section, perpendicular to the carbon graphite support, of the sample E05, prepared in Example 5.

(17) The scale shown in FIG. 9 represents 50 μm.

(18) FIG. 10 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E06, prepared in Example 6.

(19) The scale shown in FIG. 10 represents 200 μm.

(20) FIG. 11 is a photograph taken in optical microscopy, at a higher magnification, of a cross-section, perpendicular to the carbon graphite support, of the sample E06, prepared in Example 6.

(21) The scale shown in FIG. 11 represents 30 μm.

(22) FIG. 12 is a photograph taken in optical microscopy, of a cross-section, perpendicular to the carbon graphite support, of the sample E07, prepared in Example 7.

(23) The scale shown in FIG. 12 represents 50 μm.

(24) FIG. 13 is a photograph taken in optical microscopy, at a higher magnification, of a cross-section, perpendicular to the carbon graphite support, of the sample E07, prepared in Example 7.

(25) The scale shown in FIG. 13 represents 30 μm.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(26) According to the invention, reinforcements of any form, shape, are placed in at least one liquid precursor of the first oxide of at least one metal and/or at least one metalloid, then the whole is heated, that is to say the reinforcements and the liquid precursor, so as to form the first oxide by means of thermal decomposition of said liquid precursor, and to deposit the first oxide thus formed around the reinforcements thus forming the matrix.

(27) By “placed in at least one liquid precursor”, it is generally understood that the reinforcements are entirely immersed, drowned in a volume of the liquid precursor.

(28) The first oxide of at least one metal and/or at least one metalloid and the second oxide of at least one metal and/or at least one metalloid may be chosen from the compounds mentioned above.

(29) Generally, the reinforcements are fibres, and said fibres may form threads, fabrics, felts or three-dimensional structures with long fibres, or any other structure.

(30) According to the invention, it was shown that some liquid precursors may, surprisingly, when same were used in the calefaction technique, form oxide matrices by means of thermal decomposition. The preferred matrices are the matrices made of SiO.sub.2, ZrO.sub.2, TiO.sub.2 or Al.sub.2O.sub.3.

(31) Said precursors, in order to be used in the calefaction technique and in the device for implementing said technique such as the device shown in FIG. 1 must imperatively be liquid at ambient temperature.

(32) Preferably, said liquid precursors have a boiling temperature less than 300° C., preferably less than 250° C., more preferably less than 200° C. Indeed, such a boiling temperature, less than 300° C., makes it possible to optimise the decomposition of the precursor.

(33) Suitable precursors have already been cited above.

(34) The organometallics or organosilanes, more specifically the alkoxides/alcoholates of metals or of silicon are the preferred liquid precursors for obtaining oxide deposits by means of a calefaction technique.

(35) Examples of said preferred liquid precursors are shown in Table I below.

(36) The liquid precursors mentioned in Table I are molecules used in soft chemistry in order to obtain by sol/gel the sought oxides by polymerisation and polycondensation. It was highlighted, surprisingly, according to the invention that said molecules used in the sol-gel technique could play the role of liquid precursor in the calefaction technique.

(37) TABLE-US-00001 TABLE I Sought Melting Boiling matrix Precursor temperature temperature Majority cracking products SiO.sub.2 Tetraethyl −82.49° C. 168° C. SiO.sub.2, ethanol, ethylene, orthosilicate ethanal ZrO.sub.2 Zirconium tert- <25° C. 81° C. at 3 torr ZrO.sub.2, isobutane, isobutene, butoxide 2-methylpropane-2-ol TiO.sub.2 Tetra-isopropoxy 14° C. 232° C. TiO.sub.2, propene, propane, titanium isopropanol Al.sub.2O.sub.3 Aluminium tri-sec- <25° C. 206° C. at 30 torr Al.sub.2O.sub.3, 2-butanol, 2-butene butoxide

(38) Other liquid precursors may also be used, this is for example tetramethyl orthosilicate (TMOS) and aluminium acetylacetonate.

(39) In the following examples, tetraethyl orthosilicate (TEOS) and aluminium tri-sec-butoxide (aluminium butoxide) are used in a calefaction reactor in order to produce deposits of silica or of alumina on flat or fibrous structures, thus showing that the method according to the invention was indeed implemented successfully.

(40) The reinforcements and the liquid precursor may be heated at a temperature of 700° C. to 1200° C., preferably at a temperature of 800° C. to 1100° C., for a period of 5 to 120 minutes, preferably of 15 to 60 minutes.

(41) FIG. 1 illustrates a device that could to be used for implementing the method according to the invention.

(42) Said device essentially comprises two portions, i.e. a reactor or chamber 1, and a condenser, cooler, or heat exchanger 2.

(43) The wall 3 of the reactor is made of a material compatible with the liquid precursor used and that withstands the temperatures used in the method. Such a material is for example borosilicate.

(44) The substrate, consisting of the reinforcements, on which the oxide deposit is performed, is placed, disposed, supported by a structure or part 4 that has for example the shape of a cylinder or mandrel.

(45) Said structure or part 4 that supports the substrate, consisting of the reinforcements, must imperatively be made of an electrically-conductive material, such as carbon graphite. Said part 4, notably made of carbon graphite, is called support or susceptor.

(46) A sample holder 5 made of an electrically-conductive material for example made of brass, thus forming electrodes, makes it possible to hang the structure or part 4 that supports the substrate consisting of the reinforcements on which the oxide deposit is performed.

(47) The heating of the reactor 1 is resistive heating.

(48) In order to produce said resistive heating, the sample holder 5 and therefore the structure or part 4 that supports the substrate are connected to a power supply, or generator 6 by means of wires 7, and the structure or part 4 that supports the substrate, is therefore heated by Joule effect.

(49) The temperature of the substrate may be measured by an infrared radiation pyrometer 8 provided with a pyrometer sight 9, which detects without contact the rays 10 emanating from the heat source consisting of the heated substrate and that have passed through the wall 3 of the reactor at a transparent window 11.

(50) The pyrometer 8 may be connected to a programmer for regulating the power of the power supply in order to control the temperature of the substrate.

(51) The reactor 1 may comprise a conduit (not shown) for the continuous introduction of the liquid precursor into the reactor 1 and an opening provided with a valve located in the lower portion of the reactor and allowing draining of said reactor.

(52) The reactor 1 is surmounted by the condenser, cooler, or heat exchanger 2.

(53) The condenser 2, includes a coil 12, which receives the vapours, essentially containing the vapours of the precursor, from the calefaction reaction.

(54) A coolant such as water, ethylene glycol or other circulates in a sleeve 13 surrounding the coil between a supply or input of coolant 14 and output or removal of coolant 15.

(55) In the condenser 2, the vapours, from the calefaction reaction, are cooled and it is made sure that the cooling temperature is such that, essentially, only the vapours of the precursor are condensed. The liquid precursor thus recovered is then sent back into the reactor 1.

(56) Hereafter is described the operating sequence for preparing an oxide/oxide composite material according to the invention.

(57) Operating sequence:

(58) The substrate consisting of the reinforcements is disposed on the support 4 inside the reactor 1, then, possibly, sweeping of the reactor 1 is performed using an inert gas, in order to flush out the oxygen possibly present inside the reactor.

(59) The reactor is then filled with the liquid precursor 16.

(60) After having put into operation the cooling circuit 12, 14, 15 and the power supply 6, the programmer, and the pyrometer 8, the rise in temperature of the substrate is started. Subsequently, the power of the heating is increased up to boiling and reflux of the precursor 16. Said precursor, in liquid form, penetrates, infiltrates, into the voids, for example into the pores of the substrate.

(61) When the thermal decomposition or “cracking” temperature is reached (800° C. for example, for TEOS), the precursor 16 vapours are subjected to thermal decomposition or “cracking” in the substrate, which leads to the formation of the oxide and to the deposit of same within the voids, for example within the pores of the substrate, between the reinforcements, and on the reinforcements, around the reinforcements that constitute the substrate. Thus, an oxide matrix is formed. Reference can also be made to densification of the substrate. More specifically, the thermal decomposition or “cracking” is carried out at the hottest portions of the substrate.

(62) The densification front propagates from the portion of the substrate in contact with the support 4 towards the portion of the substrate remote from the support.

(63) For example, in the case of a substrate placed on a cylinder-shaped support 4, said substrate therefore comprising an inner wall in contact with the support 4 and an outer wall remote from the support, the densification front advances from the inner wall towards the outer wall.

(64) The densification front propagates into the substrate during the method at a speed that may vary between a few tens of millimetres per hour and a few centimetres per hour, depending on the maximum temperature of the substrate, sample and the nature thereof.

(65) The thermal decomposition or “cracking” gases escape from the substrate, for example via the pores not yet blocked.

(66) The gases from the reaction are removed into the upper portion of the reactor.

(67) The substrate is subsequently cooled.

(68) The method according to the invention finds its application notably in the aeronautics, space and automotive sectors.

EXAMPLES

(69) The invention will now be described with reference to the following examples, given for illustrative and non-limiting purposes.

(70) In said examples, the method according to the invention is implemented using a calefaction densification device.

(71) Thus, in order to produce oxide deposits by calefaction, a calefaction densification device otherwise known as a “Kalamazoo” device on the laboratory scale was developed.

(72) Said device is substantially similar to the device shown in FIG. 1.

(73) Said device comprises a reactor 1 consisting of a chamber 3 made of borosilicate, and may receive 200 mL of liquid calefaction precursor 16.

(74) A brass sample holder 5, also forming electrodes, makes it possible to hang the structure or part 4 that supports the substrate (i.e. the reinforcements) on which the oxide deposit is performed.

(75) The structure or part 4 that supports the substrate on which the oxide deposit is performed, must imperatively be made of an electrically-conductive material, such as carbon graphite, said part, notably made of carbon graphite, is called support or susceptor.

(76) The sample holder 5 is directly connected to a power supply 6, for example a power supply named Power Supply EA PSI 9080-100, by means of wires 7.

(77) The “Kalamazoo” reactor 1 is designed in order to be used with resistive heating. Therefore, it is by Joule effect that the support 4 is heated, from 25° C. to more than 1400° C. according to the power supplied to the system.

(78) The temperature of said support cannot be accurately monitored using thermocouples.

(79) A pyrometer 8, for example an Ircon Modline® pyrometer 3, is therefore used in order to accurately know the temperature of the heated support (indeed the support and the substrate have substantially the same temperature). This is an infrared thermometer that detects without contact the rays 10 emanating from the heat source, here the support 4 and the substrate.

(80) The support 4 reaches very high temperatures which locally causes the vaporisation and the cracking of the precursor.

(81) The reactor 1 is equipped with a Graham condenser 2 wherein ethylene glycol circulates, between an input of ethylene glycol 14 generally at 0° C. and an output of ethylene glycol 15. Said Graham condenser 2 is used in order to limit the losses of the precursor only to the cracking gases which are not condensable at 0° C.

(82) It should be noted that it is also possible to use a reactor with induction heating as described in documents [1], [2], or [3].

(83) In the following examples, which illustrate the method according to the invention (Examples 3 to 7), three substrates, were mainly used, said substrates are disposed, placed on the support and constitute the reinforcements of the matrix of the composite material prepared by the method according to the invention.

(84) Given that the “Kalamazoo” reactor operates resistively, it is essential to use as base for the support a carbon graphite bar.

(85) In order to prepare the substrates used in the examples that illustrate the method according to the invention, on the support consisting of a carbon graphite bar are placed, disposed, different fibres or fabric of fibres, which will then play the role of fibrous reinforcement in the composite material prepared by the method according to the invention.

(86) More precisely, the fibres or the fabric of fibres are wound on the carbon graphite bar.

(87) The three substrates used in the following examples that illustrate the method according to the invention were prepared by placing, disposing on the carbon bar used as support, more precisely by winding around the carbon graphite bar respectively: a thread consisting of Nextel® 610 fibres. These are fibres available from the company 3M° of 10 to 12 μm in diameter consisting at 99% of alumina; a thread consisting of Nextel® 440 fibres. These are fibres available from the company 3M® of 8 μm in diameter consisting at 98% of mullite (2% boron). Mullite is a crystalline solid, defined compound of formula (3Al.sub.2O.sub.3,2SiO.sub.2). Said fibres are thermal and electrical insulators and cannot be heated via Joule effect but only by thermal conduction, by direct contact with the carbon graphite bar; a fabric of Nextel® 440 fibres.

(88) By the method according to the invention implemented with the reactor described above, said substrates made of oxide fibres can be densified by calefaction forming an oxide matrix and therefore oxide/oxide materials can thus be produced.

(89) In the following examples 1 and 2, silica and alumina are deposited on a single carbon bar used as support and that plays here also the role of deposition substrate.

(90) The conditions of the experiments carried out in Examples 1 and 2 are described in Table II, below:

(91) TABLE-US-00002 TABLE II Temperature of the Example Substrate substrate Duration 1 Carbon graphite bar 900° C. 63 minutes 2 Carbon graphite bar 900° C. 28 minutes

EXAMPLE 1

(92) In this example, a carbon graphite bar of a diameter of 3 mm and of a height of 7 cm is used as support, and also as substrate, susceptor.

(93) The bar, immersed in 200 mL of liquid precursor, is heated to 900° C. for 63 minutes.

(94) The voltage used is 6.98 V and the intensity is 58.4 A, i.e. a power output of 407.6 W.

(95) The liquid precursor used is tetraethyl orthosilicate (TEOS) (supplier Sigma Aldrich®, purity>99%) that forms silica by thermal decomposition.

(96) The characterisation of the sample EO1 (900° C.-1 h) thus obtained is then carried out.

(97) FIG. 2 is a photograph taken in optical microscopy of a cross-section of the sample E01.

(98) A deposit can be seen in the form of a coating of silica of a constant thickness, in the order of 40 μm.

(99) The chemical characterisation of the deposit showing that this is silica is carried out by an electron probe microanalyser.

(100) No crack is present on the layer deposited at 900° C. The thickness of the layer of SiO.sub.2 deposited is homogeneous over the entire surface of the carbon bar.

EXAMPLE 2

(101) In this example, a carbon graphite bar of a diameter of 3 mm and of a height of 7 cm is used as support and as substrate, susceptor.

(102) The bar, immersed in 200 mL of liquid precursor, is heated at 900° C. for 28 minutes.

(103) The voltage used is 8.64 V and the intensity is 65 A, i.e. a power output of 561.2 W.

(104) In this example, the calefaction is performed with a precursor other than in Example 1, i.e. with aluminium tri-sec butoxide (supplier Sigma Aldrich, purity 97%) in the aim of forming alumina by means of thermal decomposition.

(105) The characterisation of the sample EO2 (900° C.-28 min) thus obtained is then carried out.

(106) FIG. 3 is a photograph taken in optical microscopy of the sample E02.

(107) FIG. 4 is also a photograph taken in optical microscopy of the sample E02, but at a higher magnification.

(108) It can be seen on said images, that a deposit in the form of an alumina coating was produced at 900° C.

(109) Said deposit is very friable, very thin, of a thickness of 40 μm, and is very cracked when it is compared with the silica deposit obtained by decomposition of TEOS in Example 1.

(110) An analysis by electron probe microanalyser was carried out and confirmed that the deposit obtained has the atomic composition Al.sub.2O.sub.3.

(111) In the following examples 3 to 7, silica is deposited on a substrate of oxide fibres or of a fabric of oxide fibres, said substrate being disposed, more precisely wound on a support consisting of a carbon graphite bar.

(112) The conditions of the experiments carried out in Examples 3 to 7 are described in Table III, below:

(113) TABLE-US-00003 TABLE III Temperature of the Example Support + substrate substrate Duration 3 Carbon bar + Nextel ® 610 900° C. 28 minutes alumina fibres 4 Carbon bar + Nextel ® 440 815° C. 28 minutes mullite fibres 5 Carbon bar + Nextel ® 440 857° C. 28 minutes mullite fibres 6 Carbon bar + Nextel ® 440 881° C. 28 minutes mullite fabric 7 Carbon bar + Nextel ® 440 1000° C.  60 minutes mullite fibres

EXAMPLE 3

(114) In this example, a SiO.sub.2/Al.sub.2O.sub.3 composite composite material is prepared, more precisely a composite material with a SiO.sub.2 matrix reinforced by alumina fibres, by the method according to the invention.

(115) A carbon bar of a diameter of 3 mm and of a height of 7 cm is used as support. Threads consisting of alumina fibres (Nextel® 610 alumina fibres, of a diameter of approximately 10 μm) are wound around said support.

(116) The bar, immersed in 200 mL of liquid precursor, is heated at 900° C. for 28 minutes.

(117) The voltage used is 8.64 V and the intensity is 65 A, i.e. a power output of 561.6 W.

(118) The liquid precursor used is tetraethyl orthosilicate (TEOS) that forms silica by thermal decomposition.

(119) The characterisation of the sample E03 (900° C.-28 min) thus obtained is then carried out.

(120) FIG. 5 is a photograph taken in optical microscopy of the sample E03.

(121) Said image illustrates first of all the fact that the alumina fibres do not thermally insulate the graphite susceptor. Therefore, the phenomenon of densification by calefaction is performed even if a thermal insulator is wound around the graphite bar and separates the hot surface from the liquid precursor.

(122) Next, as regards the microstructure, it can be seen that the silica forms a dense matrix around the alumina fibres.

(123) The silica coating around the alumina fibres shows that the method of densification by calefaction is applied to the development of oxide/oxide composites.

EXAMPLE 4

(124) In this example, a SiO.sub.2/mullite composite material, more precisely a composite material with a SiO.sub.2 matrix reinforced by mullite fibres, is prepared by the method according to the invention.

(125) A carbon bar of a diameter of 3 mm and of a height of 7 cm is used as susceptor. Threads consisting of mullite fibres (Nextel® 440 mullite fibres, of a diameter of 8 μm) are wound around said susceptor.

(126) The bar, immersed in 200 mL of liquid precursor, is heated at 815° C. for 28 minutes.

(127) The voltage used is 6.74 V and the intensity is 64.3 A, i.e. a power output of 433 W.

(128) The liquid precursor used is tetraethyl orthosilicate (TEOS) that forms silica by thermal decomposition.

(129) The characterisation of the sample E04 (815° C.-28 min) thus obtained is then carried out.

(130) FIG. 6 is a photograph taken in optical microscopy of the sample E04.

(131) Said image shows that, even at a low temperature of the substrate (815° C.), TEOS can be used in order to create a silica matrix between oxide fibres.

(132) FIG. 7 is also a photograph taken in optical microscopy of the sample E04, but at a higher magnification.

(133) In said sample, prepared at a low temperature of the substrate, it is possible to note that the silica matrix is perfectly infiltrated into the fibrous mullite structure, and that it is dense, homogeneous and free of cracks. It can also be seen that the matrix does not contain carbon. The matrix surrounding the mullite fibres is entirely constituted of non-cracked silica.

EXAMPLE 5

(134) In this example, a SiO.sub.2/mullite composite material, more precisely a composite material with a SiO.sub.2 matrix reinforced by mullite fibres, is prepared by the method according to the invention.

(135) A carbon bar of a diameter of 3 mm and of a height of 7 cm is used as support. Threads consisting of mullite fibres (Nextel® 440 mullite fibres, of a diameter of 8 μm) are wound around said support.

(136) The bar, immersed in 200 mL of liquid precursor, is heated at 857° C. for 28 minutes.

(137) The voltage used is 7.25 V and the intensity is 66.9 A, i.e. a power output of 485 W.

(138) The liquid precursor used is tetraethyl orthosilicate (TEOS) that forms silica by thermal decomposition.

(139) The characterisation of the sample E05 (857° C.-28 min) thus obtained is then carried out.

(140) FIG. 8 is a photograph taken in optical microscopy of the sample E05.

(141) Said image shows that, when a temperature of the substrate, i.e. 857° C., greater than the temperature used for preparing the preceding sample E04 in Example 4, is used, the prepared sample E05, has a matrix that is more cracked than the matrix of Example E04 with wide pores.

(142) FIG. 9 is also a photograph taken in optical microscopy of the sample E05, but at a higher magnification.

(143) Said image shows that, within the mullite fibres, the formed silica, dense, has a few cracks and a few small pores but no carbon. Therefore, the deposit obtained is exclusively constituted of SiO.sub.2.

EXAMPLE 6

(144) In this example, a SiO.sub.2/mullite composite material, more precisely a composite material with a SiO.sub.2 matrix reinforced by mullite fibres, is prepared by the method according to the invention.

(145) A carbon bar of a diameter of 3 mm and of a height of 7 cm is used as support. Threads consisting of mullite fibres (Nextel® 440 mullite fibres, of a diameter of 8 μm) are wound around said support.

(146) The bar, immersed in 200 mL of liquid precursor, is heated at 881° C. for 28 minutes.

(147) The voltage used is 7.80 V and the intensity is 65.7 A, i.e. a power output of 512 W.

(148) The liquid precursor used is tetraethyl orthosilicate (TEOS) that forms silica by thermal decomposition.

(149) The characterisation of the sample E06 (881° C.-28 min) thus obtained is then carried out.

(150) FIG. 10 is a photograph taken in optical microscopy of the sample E06.

(151) Said image shows that, when a temperature of the substrate of 881° C. is used, the fibres are coated with a non-cracked silica matrix that has wide pores.

(152) FIG. 11 is also a photograph taken in optical microscopy of the sample E06, but at a higher magnification.

(153) At higher magnification, it can be seen that the silica is infiltrated properly into the intra-fibre area. There is no decohesion between the fibres and the matrix, but if wide pores are present, they are not easily filled by the matrix. In addition, at the extreme limit of the silica deposit, it is possible to see a clearer, very fine, area that corresponds to a carbon deposit of thickness 27 nm. This means that when a temperature of the substrate of 881° C. is used, which is higher than the temperature of the substrate used in the preceding examples, for an identical heating time, carbon may be formed. Said carbon comes from the maturing of the precursor, which is more rapid when the temperature is higher.

EXAMPLE 7

(154) In this example, a SiO.sub.2/mullite composite material, more precisely a composite material with a SiO.sub.2 matrix reinforced by a fabric of mullite fibres, is prepared by the method according to the invention.

(155) A carbon bar of a diameter of 3 mm and of a height of 7 cm is used as support. A mullite fibre fabric (Nextel® 440 mullite fibres, of a diameter of 8 μm) is wound around said support in order to experiment the calefaction of oxides on a substrate including a large number of fibres.

(156) The bar, immersed in 200 mL of liquid precursor, is heated at 1000° C. for 60 minutes (1 hour).

(157) The voltage used is 7.62 V and the intensity is 76.2 A, i.e. a power output of 580 W.

(158) The liquid precursor used is tetraethyl orthosilicate (TEOS) that forms silica by thermal decomposition.

(159) The characterisation of the sample E07 (1000° C.-1 h) thus obtained is then carried out.

(160) FIG. 12 is a photograph taken in optical microscopy of the sample E07.

(161) FIG. 13 is also a photograph taken in optical microscopy of the sample E07, but at a higher magnification.

(162) The experiment carried out in this example was carried out at a high temperature, i.e. 1000° C., for a substantial time, i.e. 1 h.

(163) It is possible to note on the images of FIGS. 13 and 14 that the silica matrix has properly infiltrated the fibres. However, said matrix has two types of porosity, that is to say a small-size porosity close to the fibres, and a large-size porosity as soon as there are no more fibres in the areas of large macropores. There is no cracking at the interface between the fibres and the matrix. The presence of a thick and shiny border of carbon can be seen within the silica matrix around the periphery of the wide pores. Said carbon is due to the excessive maturing of the TEOS precursor and to the decomposition of the by-products thereof such as ethanol and the other carboneous molecules. The entire fabric was densified by the silica over a thickness estimated at 3 mm. Thus, it is possible to make thick composites in the order of a few mm by the method according to the invention.

(164) Conclusions of the Examples

(165) The examples presented above provide the proof that oxide/oxide composite materials with silica and alumina matrices can be prepared by the method according to the invention.

(166) However, the man skilled in the art will understand that the method according to the invention may also be used to prepare composite materials with matrices consisting of any other oxide such as for example, zirconia or titanium oxide.

(167) The man skilled in the art will indeed easily know how to determine the suitable precursor for forming the oxide chosen to constitute the matrix.

(168) In the same manner, the examples presented above provide the proof that oxide/oxide composite materials with reinforcements that are mullite or alumina fibres can be prepared by the method according to the invention.

(169) However, the man skilled in the art will understand that the method according to the invention may also be used to prepare composite materials with reinforcements consisting of silica fibres or any oxide fibre.

(170) Similarly, the reinforcements may be in any form for example, in the form of threads, fabrics or felts or even of three-dimensional structures with long fibres.

(171) The resistive heating reactor used in the examples may only be used to densify substrates that can be heated by Joule effect. Therefore, densifying fibres made of oxides is generally only possible for a low thickness of substrate, in the order of a few millimetres (see Example 7).

(172) The use of induction heating makes it possible to directly heat substrates made of oxides to be densified by freeing of the carbon susceptor and of the cylinder shape of the structure.

REFERENCES

(173) [1] HOUDAYER M., et al., “Procédé de densification d′une structure poreuse”: EP-A1-0 081 409. [2] CONNORS D. F. Jr., “Partially densified carbon preform”: WO-A1-99/40043. [3] DAVID P., et al., “Procédé de densification d'une structure poreuse par du nitrure de bore et structure poreuse densifiée par du nitrure de bore”: FR-A1-2 712 884.