GB2141418A

GB2141418A - A process for the production of carbon-containing materials having ultrafine grains

Info

Publication number: GB2141418A
Application number: GB08414925A
Authority: GB
Inventors: Jacques Maire
Original assignee: Carbone Lorraine SA
Current assignee: Mersen SA
Priority date: 1983-06-15
Filing date: 1984-06-12
Publication date: 1984-12-19
Also published as: IT8421311A0; FR2548174B1; GB2141418B; IT1180180B; DE3422388C2; FR2548174A1; GB8414925D0; DE3422388A1

Abstract

The present invention relates to a process for the production of carbon-containing materials having ultrafine grains. This process is characterised in that a chemical deposition in the vapour phase is effected on all or part of a silica aerogel. The latter may be a self-supported block or may fill the pores of a porous material eg carbon foam, polycrystalline graphite or carbon fibre based substrate. Uses: the densification and improvement of the ablation-and oxidation-resistance of carbon-containing materials, intended in particular for the production of nozzles, brakes, and elements for chemical engineering.

Description

SPECIFICATION A process for the production of carbon-containing materials having ultrafine grains It is well established in the art that industrial carbons and graphites and carbon fibre/carbon matrix compounds are porous materials having a pore size in the range of from several tenths of a micron to one hundred microns or more.

This porosity creates disadvantages for certain uses of these materials. These include its use for nozzles, oxidation resistant materials, materials for fused-salt reactors, and heat exchangers etc.

The waterproofing or densifying of these porous materials is limited either by the closure of the narrow necks of pores if the process is a chemical deposition in the vapour phase (henceforth abbreviated to CDVP) or by the number of operations if the process is a liquid impregnation.

It is, moreover, known that before commercialisation compressed carbon black is generally in the form of aggregates. These aggregates may be agglomerated by a binder, such as pitch, or densified by a CDVP, but they produce a porous material similar two industrial carbons.

In contrast, if acetylene black drawn from the outlet of the reactor which has not been compacted is used as a starting material, a homogenous material may be obtained by isostatic or mono directional compression, the pores of which are submicronic, not only when the openings of the pores are measured, but also in the pores themselves (this is essentially due to the three dimensional texture of the particles of this black). This homogeneous material having ultrafine dimensions is densified well by carbon CDVP and produces a dense carbon which, even though it has good resistance to ablation, is not sufficiently resistant to thermal shock.

In his French patent application filed on 10th September 1981, Publication No. 2512441 for a "Process for the production of low-density carbon having ultrafine homogeneous porosit", the Applicant claims to be able to produce a so-called "aerocarbon" material.

Aerocarbon is obtained by producing a suspension of carbon black particles in a liquid which is subsequently removed under hypercritical conditions.

If the aerocarbon is densified well by CDVP, producing a material having ultrafine grains, it is not always easy to produce it having a homogeneous density, especially when a fibrous substrate is to be impregnated with black.

The main object of the present invention is to produce carbon-containing materials having untrafine grains and a homogeneous density which do not suffer from the disadvantages of this type of material and/or are easier to use.

Accordingly the present invention, provides a process for producing carbon-containing materials having ultrafine grains consisting of the CDVP of carbon, characterised in that the carbon is completely or partially deposited on a silica aerogel.

The silica aerogels are generally obtained by producing a silica gel impregnated with solvent, and removing the solvent under hypercritical conditions.

A silica aerogel may, for example, be obtained in the following manner: a silica gel impregnated with solvent is produced by hydrolysing a methyl orthosilicate diluted in methanol; the methanol is removed under hypercricitcal conditions: at a pressure of more than 78 bars and at a temperature of more than 240"C (these values are the critical parameters of the methanol).

Figure 1 shows the cycle (indicated by the arrow) for producing this aerogel using gel impregnated with methanol.

As the silica particles are reticulated among themselves when the gel is formed, the gel remains in place when the solvent is removed and as the solvent is removed under hypercricital conditions, a silica aerogel is obtained which has neither contracted not fissured.

This aerogel has a homogeneous density and an ultrafine microporosity (from 10 to 1000 Â ) which is favourable to a carbon CDVP.

Nevertheless, the conditions of this CDVP should be adapted so that densification takes place in the mass of the entire aerogel. In general, the process is carried out at a more elevated pressure and at a lower temperature than those used under conventional CDVP conditions for coarser substrates (such as polycrystalline graphite, fibrous substrates).

Figure 2 shows the densification of a silica aerogel by carbon CDVP using methane under different conditions, as a function of time: curve (1) conventional conditions: pressure (P) ; 10 mbtemperature (T) = 1000"C curve (2) P = 100 mb; T = 800"C curve (3) P = 1 bar ; T = 700"C.

According to the present invention, the aerogel may be a self-supported block or be formed in the pores of porous materials, such as polycrystalline graphites, foams, fibrous substrates (felts, carbon fibre/carbon matrix composites etc.) for example.

In this latter case, the porous material is impregnated with the liquid mixture which forms the silica gel and the solvent of the mixture subsequently removed under hypercritical conditions: the pores of the material are then filled with aerogel. The resulting material is densified well by carbon CDVP.

According to another variant of the present invention, the composition of the porous material may be created in the liquid mixture which forms the silica gel, the solvent being subsequently removed under hypercritical conditions.

This is, for example, the case with certain fibrous substrates which may be formed in the liquid mixture which produces the silica gel by placing loose fibres therein or stacking deposits of fibres or fabrics therein, filtering the excess of liquid to densify in fibres and subsequently removing the solvent under hypercritical conditions. A fibrous substrate impregnated with aerogel is thus obtained.

Like the preceding material, this substrate is densified well by carbon CDVP.

A last variant according to the present invention consists of carrying out the carbon CDVP on a silica aerogel containing carbon black. The carbon black is obtained by dispersing particles of carbon black in the liquid mixture which forms the silica gel and forming the silica gel by removing the solvent under hypercritical conditions.

During this latter operation, the particles of carbon black are maintained in place by the silica gel thereby producing a homogeneous distribution of carbon black.

As in the preceding case, this silica gel which contains the carbon black may be a self-supported block or may be formed in the pores of the porous materials.

In all cases it can be seen that the silica does not react during the CDVP and that the pores are completely filled.

The materials which are ultimately obtained are highly densified, enable load and calories to be transferred in an excellent manner, have very good oxidation-resistance and good waterproofing.

In many cases they may be used as such, nevertheless, for certain uses, the presence of silica may be disadvantageous. Should this be the case, it may be removed by heat treatment at 2.700 C or even better by heat treatment at 2,8000C under chlorine. These treatments do not substantially modify the texture and structure of the carbon.

Moreover it should be noted that after these treatments, the pores created by removing the silica may be filled, by carrying out a CDVP of the final carbon.

The following Examples illustrate the present invention but do not limit the scope thereof.

Example 1 A self-supported silica aerogel tube, 100/80 mm in diameter and 100 mm in height, is obtained by hydrolysing a methyl orthosilicate diluted in an aqueous solution of methanol and removing the methanol under hypercritical conditions at a (temperature of more than 240"C and at a pressure of more than 78 bars).

The density of this block is 0.2.

A carbon CDVP is carried out by cracking the methane at a pressure of 1 bar, at 700"C.

After about 1100 hours, a carbon-containing material is obtained which has a density of 1.6, the oxidation-resistance properties of which are similar to those of vitrious carbon.

Example 2 Atridirectional carbon fibre substrate having a density of 0.7 is impregnated with a silica aerogel by placing it in an aqueous methanol solution containing a methyl orthosilicate, hydrolysing the latter to form the silica gel and removing the methanol under hypercritical conditions.

This substrate which has been impregnated with aerogel then undergoes a carbon CDVP under the following conditions; gas used: methane temperature: 700"C pressure: 1 bar After 400 hours, a material is obtained which has a density of 1.7.

By way of comparison, Figure 3 shows, as a function of time, the densification of this same substrate: - without any preliminary impregnation before the CDVP (curve 1) under conventional conditions, -with preliminary impregnation with acetylene black which has been simply dried (curve 2), - with preliminary impregnation with aerocarbon (curve 3) and - with preliminary impregnation with aerogel according to the present invention (curve 4).

It can be seen that, without preliminary impregnation or with impregnation with acetylene black, the densification is limited to 1.5 after about 1000 hours, whereas a density of 1.7 is obtained after a relatively short period of time (400 hours) with preliminary impregnation with aerogel, this showing the great interest in the process according to the present invention.

Figure 4 shows a micrograph of the material obtained according to this Example. It can be seen that the empty octets of the 3D are all completely filled with aerogel and pyrocarbon.

If a carbon CDVP is used: - without preliminary impregnation these octets are empty, - with preliminary impregnation with acetylene black which has been simply dried, there are small piles of densified black, - with preliminary impregnation with aerocarbon, the octets are to a greater or lesser extent.

It can also be seen in Figure 4 that there is virtually no discontinuity between the fibres and the aerogel and that the load and calories can be transferred equally well from one strand to another when used under stress at elevated temperature.

The materials which are obtained according to this Example may have numerous uses and particularly for the production of nozzles.

Example 3 A carbon felt having a density of 0.3 is impregnated with aerogel in the same manner as the fibrous 3D substrate of the preceding Example, and then undergoes a carbon CDVP under the same conditions as those specified in the preceding Example.

After a period of 350 hours, the density of the felt is 1.7 whereas 600 hours are necessary to obtain this same density without preliminary impregnation with aerogel (the CDVP is effected at 1,000 C under a pressure of 10 mbars).

Figures 5 and 6 are micrographs of carbon felt which has been densified by CDVP under the above-specified conditions, with and without preliminary impregnation with aerogel respectively. Figure 5 shows a submicronic carbon isotrope without any particular deposit on the fibres. Figure 6 shows the usual aspect of a carbon-carbon composite.

The material which is obtained according to this Example has many uses and, particularly in the production of brakes and nozzles.

Example 4 Polycrystalline graphite is impregnated with a silica aerogel in the same manner as the materials in Examples 2 and 3, and then undergoes a carbon CDVP under the same conditions as those used in these Examples.

The material which is finally obtained has improved oxidation-resistance and tightness.

This material also has numerous uses, particularly in the production of elements for chemical engineering.

In general the process according to the present invention enables materials having markedly improved resistance to ablation and oxidation and elevated density to be obtained which are unobtainable by conventional densification methods. High pressure-high temperature impregnation (1000 bars; 1000"C) alone enables elevated densities to be attained, at the price of a very substantial investment and even then only for pieces with restricted limited dimensions.

It should be noted that the process according to the present invention 1) may be repeated several times on a porous material to correct possible defects, 2) may be used to perfect the densification of porous materials which have already been partially densified to a greater or lesser extent by other processes, such as the formation of aerocarbon and/or CDVP.

Claims

1. A process for the production of carbon-containing materials having ultrafine grains consisting of a chemical deposition in the vapour phase of carbon, characterised in that the chemical deposition in the vapour phase of carbon is completely or partially carried out on a silica aerogel.

2. A process according to claim 1, characterised in that the silica aerogel contains carbon black.

3. A process according to claim 1 or claim 2, characterised in that the silica gel is a self-supported block.

4. A process according to claim 1 or claim 2, characterised in that the silica gel is formed in the pores of porous materials.

5. A process according to claim 4, characterised in that the porous material is a carbon foam, a polycrystalline graphite or a carbon fibre-based substrate.

6. A process according to claim 5, characterised in that the carbon fibre-based substrate is a 3D.

7. A process according to claim 5, characterised in that the carbon fibre-based substrate is a felt.

8. A process according to claim 1 or claim 2, characterised in that a carbon fibre substrate is formed in the liquid mixture which forms the aerogel and which subsequently produces it.

9. A process according to any one of the preceding claims, characterised in that the chemical deposition in the vapour phase of carbon is carried out at a more elevated pressure and at a lower temperature than those conventionally used.

10. Carbon-containing materials having ultrafine grains characterised in that they are produced according to any one of the preceding claims.

11. A process for the production of carbon-containing materials substantially as herein described with reference to any one of the specific examples.