FR2479173A1 - Accelerated densification of porous carbon - by impregnating with decomposable hydrocarbon, then heating under pressure - Google Patents

Abstract

Accelerated densification of a porous carbon substrate (A) comprises decomposing, in the presence of (A), pitch, acenaphthylene (I), phenanthrene or their mixts. at high temps. and pressure. Decompsn. is carried out in a closed reactor, allowing release of hydrogen as it forms, the walls of the reactor being sufficiently flexible to adapt to the vol. of the liq. phase. Pref. pitch, or other hydrocarbon contg. free C atoms is used. Pref. a mixt. of 5-40(30) wt.% (I) and pitch is used with C:H atom ratio over 1.4, contg. below 0.1% quinoline insolubles; about 20% toluene insolubles; below 0.2% sulphur and below 0.1% each oxygen and nitrogen. This material is made by separately dry-grinding (I) and the pitch to particle size below 100 microns, mixing the powders, then degassing at about 140 deg.C with continuous agitation until residual pressure in the reactor is about 5 mm. An appts. for the process is also claimed. The dense carbon articles produced have high mechanical strength and abrasion resistance at high temp., and are useful as electrodes, car or aircraft brakes, and medical prostheses. Materials of high free carbon content can now be used without problems of warping the substrate walls.

Description

The present invention which results from the work of Messrs Jean JAMET, Jean-Jacques POUPEAU, Claude
LEPENNEC and Madame Jacqueline OMNES, from the Office
National Aerospace Studies and Research, and
Commissariat à l'Energie Atomique, relates to the manufacture of compact carbonaceous materials by densification of porous substrates, this being done by decomposition of hydrocarbon products, in particular pitches, at high temperature and high pressure.

The term “densification” is understood to mean an operation consisting in supplying carbon to a porous substrate and it is in this sense that the word will be used in the rest of this description.

The most recent technique for developing compact carbonaceous materials, such as carbon-carbon composites for example, preferably uses pitches under high pressure. The latter, essentially consisting of carbon and hydrogen, have a softening point around 70-1000C. Beyond that, they liquefy and remain stable until around 350-400 C: from this temperature, they begin to decompose with release of hydrogen and volatile saturated hydrocarbons, while a mesophase is formed in the mass liquid.

This is in the form of small spheres, similar to liquid crystals, which grow and end, by coalescence, to invade the entire liquid phase. Once the reaction is complete, the substance to be densified undergoes a graphitization treatment at around 26000C and the cycle is repeated until the desired density is obtained.

In order to increase the carbon yields, this transformation was carried out in crimped capsules and under high pressure.

Since the capsules used to date are very slightly permeable, the gases produced during the reaction can cause nucleation within the material, this nucleation moving towards the walls and ultimately giving a certain porosity to the product obtained. In addition, the number of cycles necessary to obtain the final product of given density may be greater or less.

 The object of the present invention is precisely a method and a device making this densification faster and more economical by increasing the yield of the densification reactions and by reducing the number of cycles necessary to arrive at a final product of given density.

According to the essential characteristic of the process which is the subject of the invention, this process, of the kind which consists in decomposing a hydrocarbon substance, in particular a pitch, in the presence of the substrate to be densified, under high pressure and at high temperature, is characterized in that that said decomposition is carried out in a reactor allowing the elimination of the hydrogen released during the reaction as it is produced; on the other hand, the walls of said reactor have the flexibility necessary so that the internal volume of the latter adapts at each instant, under the effect of the pressure applied, to the volume of the liquid phase which it contains.

 The elimination of hydrogen as it is produced promotes both the decomposition of the hydrocarbon substance and that of volatile hydrocarbons. The yield of the decomposition reactions is thus improved by reducing the pore volume caused by the formation of gas nucleations within the material: the manufacturing costs are therefore reduced by reducing the number of cycles required while obtaining a more homogeneous product.

 The elimination of hydrogen can be done in two ways: either by trapping it using a substance in which this gas has a high solubility, or by making it diffuse through the walls of the reactor, if the material constituting these has sufficient permeability, or even by combining these two phenomena.

 As for the flexibility of the reactor walls, it makes it possible to further improve the yield of the reactions by making it possible to work under very high pressure without risk of rupture for the reactor since the external pressure is constantly balanced by that of the incompressible liquid mass which it contains.

The present invention also relates to a device for implementing the method. According to the preferred embodiment, this device is characterized in that it comprises an aneroid capsule whose walls, made of a material permeable to hydrogen, are thin and wavy.

Indeed, to obtain a good permeability of the reactor vis-à-vis hydrogen, it is not only necessary to choose a material permeable to this gas, but moreover it is very advantageous to increase as much as possible the surface / thickness ratio of the walls of it, hence the shape chosen. In addition, this shape makes the capsule flexible enough so that it can deform during treatment, so that the effect of the pressure applied without the risk of cracking. Finally, such structures have the advantage of being commercially available in a very wide range of dimensions and materials, which reduces the cost of manufacturing such capsules.

Other characteristics and advantages of the invention will appear more clearly and the implementation of the method will be better understood on reading the description which follows, given by way of purely illustrative and nonlimiting example, with reference to the accompanying drawings. wherein
- Figure 1 shows a sectional view of the aneroid capsule used;
- Figure 2 shows a sectional view of the vacuum device of the capsule of Figure 1;
- Figure 3 shows, in logarithmic scale, the curves giving the mass flow rate of hydrogen (in grams / second) as a function of time (in hours) for two types of capsules
- Figure 4 shows the curves giving the increase in density of the substrate after each cycle for three different types of capsule.

 FIG. 1 represents an aneroid capsule 1 with thin and wavy walls 2. This shape makes it possible to considerably increase the surface / thickness ratio of the walls of the capsule, making it more permeable to hydrogen provided, of course, that the constituent material of the capsule allows the diffusion of this gas. Good permeability is obtained by choosing a metal such as nickel or stainless steel as the constituent material of the capsule. If the economic aspect is not important one can use other materials such as palladium or an alloy such as palladium-silver for example.

In FIG. 1, we can also see the closing device 3 constituted by a cover 4 welded to the capsule 1, the seal being ensured by means of the ball 5 which is applied to the cone 6 by means of the screw 7, the latter having a flat 8 making it possible to create a vacuum. This latter operation is also facilitated by the central hole 7A which passes through the screw 7.

The implementation of the method is done as follows: first placing the product to be densified 9, for example a carbon fiber reinforcement, in the capsule 1 not closed. This is then placed in an enclosure where a vacuum is produced, then the capsule is filled with pitch 10, previously liquefied, the residual pressure in the enclosure being of the order of 5 to 10 mm Hg. Indeed, for the pitch to penetrate the substrate well, it is necessary to carry out this operation under vacuum and to leave the liquid pitch in contact with the substrate for a sufficient time. It is then allowed to cool and it is brought back to atmospheric pressure.

The process, used essentially for densifying carbon-carbon composites, can naturally be applied to other substances and the starting carbon substrate can be either a carbon fiber reinforcement, or any carbon product whose density one would like to increase. .

 The cover 4 is then welded to the capsule 1 and a vacuum is produced by means of the system shown in FIG. 2. A cap It is placed at the upper part of the capsule 1, the seal being ensured by means of the seal 12 held by the screws 13. On the other hand, the screw 7 is tightened using a telescopic key 14 the latter being operated by the control system 15: the seal between the latter and the cap ll is ensured by means of the seals 16. A duct 17 connected to a vacuum pump, not shown, passes through the system 15.

When a vacuum is created in the capsule 1 and in the interior space 18 of the cap 11, the control system 15 is put into action and the screw 7 is tightened to lock the ball 5 on the cone 6. I1 remains a slight space empty at the upper part of the capsule, the residual pressure also being of the order of 5 to 10 mm Hg.

 This vacuum device is then removed and the capsule is placed in a high pressure enclosure which is heated to 2000C; a level must be observed at this temperature to ensure that the pitch is well melted.

Then the temperature gradually rose to around 600-7000C while the capsule was subjected to a high pressure of argon (of the order of 700 to 1000 bars). Under the effect of this pressure, the capsule crashes, which removes the empty space created previously. The interior of the capsule is then completely filled with the liquid phase. It is therefore possible to work at very high pressures (up to 2000 bars, for example, if the installation allows it) without risk of rupture for the capsule since the external pressure will always be balanced by that of the liquid that it contains.

An argon sweep is necessary to extract the hydrogen from the enclosure, so that the partial pressure thereof outside the capsule is always as low as possible. The continuous analysis of the gases leaving the high pressure enclosure makes it possible to know the hydrogen flow rate at all times.

This is represented, on a logarithmic scale and in grams / second, as a function of time in hours and of the temperature rise in FIG. 3 for two types of capsule of the same volume allowing a comparison between - a straight capsule, with walls stainless steel beams
ble (curve 21) of the type used in the art
anterior, and - an aneroid capsule with thin, wavy walls in
nickel (curve 22) according to the present invention.

This figure also shows the evolution
tion of the temperature as a function of time (court
be23).

 The experiment described here was carried out under a pressure of 700 bars. As for the pitch used, it is a coal pitch with a softening point (KS or Ring-Ball) at 900C, and containing 30% of free carbon (phase a); in fact, the latter favors the densification reaction. On the other hand, this pitch has a carbon yield of 40% and a carbon / hydrogen atomic ratio equal to 1.89.

However, coal pitches contain sulfur, which leads to two kinds of disadvantages: first, there is a risk of deteriorating the capsules, and secondly, around 13000C the sulfur leaves suddenly, which causes a sudden withdrawal of the matrix and therefore requires certain precautions during the graphitization treatment.

You can also use petroleum pitches, which contain very little sulfur, but it is in your interest to add a product such as acenaphthylene for example: it is an aromatic hydrocarbon of chemical formula C12H8, which mixed to the pitch of petroleum in the proportion of 30%, gives a eutectic which melts at 500C (instead of 2000C as before) and which, moreover, has an almost zero ash rate.

 It is understood that it is possible to use substances other than pitches, provided that the atomic ratio C / H is between 1.4 and 1.9. Pure products such as acenaphthylene or phenanthrene (aromatic compound of formula C14H10) may also be suitable, but they are unfortunately very expensive.

 When reading FIG. 2, it can be seen that the aneroide nickel capsule significantly increases the yield of the reaction compared to the straight stainless steel capsule: with the first, the diffusion of hydrogen begins much earlier, as soon as the temperature reaches a value close to 4000C at which the pitch begins to decompose, while with the second, nothing happens below 5000C. In addition, at a given instant, the hydrogen flow rate is approximately 100 times stronger with the nickel aneroid capsule than with the straight stainless steel capsule.

After about 5 hours the flow of hydrogen greatly decreases or ceases, and the experiment is stopped; the substrate is then extracted from the capsule and it is subjected to a heat treatment at 26000C to make the carbon crystallize in the graphite system.

The operations are then repeated, repeating the cycle as many times as necessary to obtain the desired final density.

FIG. 4 indicates the density of the product obtained after each densification cycle for the same substrate of initial density 1.12 with three types of capsule - straight stainless steel capsule (curve 31) according to the prior art
laughing - stainless aneroid capsule (curve 32) - nickel aneroid capsule (curve 33i.

It can be seen that, for the same starting product, the aneroid nickel capsule makes it possible to obtain a final density of 1.90 after three cycles whereas it takes four with the straight stainless steel capsule.

 On the other hand, the comparison of curves 31 and 32 shows, for the same constituent material of the capsule, the improvement brought by the structure with thin and wavy walls, compared to the structure with smooth walls.

The comparison of curves 32 and 33 shows that, for the same structure of the capsule, nickel is more effective than stainless steel.

 Curves 41 and 43 give the density gain for each cycle with the straight stainless steel capsule and the nickel aneroid capsule respectively.

 These curves show that after the first cycle, the density increase is much greater with the aneroide nickel capsule (curve 43) than with the straight stainless steel capsule (curve 41). During the following cycles, the gain in density decreases and eventually vanishes when the substrate reaches its maximum density.

 It can therefore be seen that the invention does indeed achieve the aim which is the rapid and economical production of compact carbonaceous materials. In addition, the elimination of the gases produced makes it possible to considerably attenuate any gas nucleation within the material, a source of porosity which can have a detrimental influence on the mechanical properties.

It goes without saying that the present invention is not limited to the example described above, and that other embodiments and implementation are possible.

The elimination of hydrogen can also be done by trapping using a titanium sponge or with palladium black, two materials in which hydrogen has a high solubility.

 Other forms of reactor are also possible and one can envisage flattened and circular capsules, always having corrugated walls, of the kind of those used in aneroide barometers.

The applications are numerous and varied, in particular for the manufacture of compact carbonaceous materials of any shape and size: these and more especially carbon-carbon composites are practically the only ones usable in the fields where a great mechanical resistance and a high abrasion resistance at high temperatures are required. Another example is the manufacture of electrodes, brakes for cars or planes, possibly medical prostheses since carbon is compatible with living tissue.

Claims (10)

1. Process for the accelerated densification of a porous carbonaceous substrate (9) of the type which consists in decomposing a hydrocarbon substance (10), in particular pitch, in the presence of said substrate under high pressure and at high temperature, characterized in that that said decomposition is carried out inside a closed reactor allowing the elimination of the hydrogen released during the reaction as and when it is produced, the walls of said reactor having the flexibility necessary for the interior volume of it adapts at each instant, under the effect of the pressure applied, to the volume of the liquid phase it contains. 2. Method for the accelerated densification of a porous carbon substrate according to claim I, characterized in that the hydrocarbon substance to be decomposed is a pitch containing free carbon and having an atomic ratio C / H of between 1.4 and 1, 9.  3. Method for the accelerated densification of a porous carbon substrate according to claim 1, characterized in that the hydrocarbon substance to be decomposed is an aromatic hydrocarbon chosen from the group comprising acenaphthylene and phenanthrene. 4. Method for the accelerated densification of a porous carbon substrate according to claim 1, characterized in that it comprises at least one cycle comprising the following steps - the porous substrate is placed in a capsule (1) ser front of the reactor, the capsule being placed in an enclosure where  vacuum, we run the hydrocarbon substance born (10) to decompose, previously liquefied, in  the capsule (1), - allow to cool, reduce to atmospheric pressure  risk and the capsule (1) is taken out of the enclosure, - the capsule (1) is closed by a cover (4) which is  soda, - we vacuum in said capsule (1) and place this in a high pressure enclosure where brings the temperature to around 200 ° C., then increases the pressure inside said high pressure enclosure up to a gold value from 700 to 1000 bars, while the temperature is  range up to around 600-700 C, - We analyze the gases from the enclosure at high pressure sion and the reaction is stopped when it does not emerge  practically more hydrogen and then the substrate is  subjected to a graphitization treatment at a temperature  erasure of around 26000C 5. Process by the accelerated densification of a porous carbonaceous substrate (9) according to claim 1, characterized in that the porous carbonaceous substrate (9) at the start is a reinforcement made of carbon fibers 6 A method for the accelerated densification of a porous carbon substrate according to claim 12 characterized in that the elimination of hydrogen is done by trapping it  7. Method for the accelerated densification of a porous carbon substrate according to claim 6, characterized in that a hydrogen sponge of titanium or palladium black is used as the trap 8. Method for the accelerated densification of a porous carbon substrate (9) according to claim 1, characterized in that the elimination of the hydrogen is effected by diffusion of the latter through the walls of the reactor .  9. Device for the accelerated densification of a porous carbon substrate (9) for the implementation of the method according to claim 1, characterized in that it is constituted by an aneroid capsule (1) whose walls (2), made of a material permeable to hydrogen, are thin and wavy.  10. Device for the accelerated densification of a porous carbon substrate (9) according to claim 9, characterized in that said aneroid capsule (1) is constituted, at least for its corrugated part, by a material such as stainless steel , nickel, palladium or silver-palladium.