THERMAL INSULATION MATERIALS SUITABLE FOR USE AT HIGH TEMPERATURES, AND PROCESS FOR MAKING SAID MATERIALS

Abstract

A process for making a thermal insulation material based on carbon and which includes carbon fibers, suitable for use at temperatures above 1,500° C. The process includes providing carbon fibers with embedded carbon black particles; cutting or milling said carbon fibers to obtain short carbon filaments; preparing a slurry by introducing the short carbon filaments in a liquid phase that includes a binder capable of forming a carbon residue upon pyrolysis under non-oxidizing conditions; casting the slurry into a mold capable of separating the slurry into a wet green body and a liquid phase; and drying and heat treating the wet green body to obtain a thermal insulation material.

Claims

1-17. (canceled)

18. A process for making a thermal insulation material comprising carbon fibers, the process comprising: providing carbon fibers with embedded carbon black particles; cutting or milling the carbon fibers to obtain short carbon filaments with an average length not exceeding about 2,000 μm; preparing a slurry by introducing the short carbon filaments in a liquid phase comprising a binder capable of forming upon heat treatment at a temperature of at least 700° C. under non-oxidizing conditions a carbon residue representing 10% or more of the initial mass of the binder; casting said slurry into a mold to separate the slurry into a wet green body and a liquid phase; and drying and heat treating the wet green body to obtain a thermal insulation material.

19. The process of claim 18, wherein said drying and heat treating comprises drying and solidifying the binder, and conducting pyrolysis under non-oxidizing conditions at a minimum temperature of 700° C. to transform the binder into a carbon residue.

20. The process of claim 19, wherein conducting the pyrolysis is carried out at a temperature of at least 900° C.

21. The process of claim 20, further comprising, after conducting the pyrolysis, conducting a heat treatment at a temperature above 2,000° C. under non-oxidizing conditions.

22. The process of claim 18, wherein the carbon filaments have an average length of between about 200 μm and about 1500 μm.

23. The process of claim 18, wherein the carbon yield of the binder is at least 20%.

24. The process of claim 18, wherein the mass fraction of the carbon black particles in the carbon fibers is between 10% and 35%.

25. The process of claim 18, wherein the binder comprises an agent selected from the group consisting of phenolic resin, sugar, and starch.

26. The process of claim 18, wherein the liquid phase comprises water.

27. The process of claim 18, wherein the carbon fibers have a mean diameter not exceeding 15 μm.

28. The process of claim 18, wherein the carbon fibers have a mean diameter of between 1 μm and 10 μm.

29. A thermal insulation material, formed by the process of claim 18.

30. The thermal insulation material of claim 29, wherein the thermal insulation material comprises a rigid board.

31. The thermal insulation material of claim 29, wherein the thermal insulation material comprises a rigid three-dimensional shape.

32. The thermal insulation material of claim 29, wherein the thermal insulation material has a thermal conductivity at 1,000° C. that does not exceed 0.20 W/m.Math.K.

33. The thermal insulation material of claim 29, wherein the thermal insulation material has a thermal conductivity at 1,700° C. that does not exceed 0.40 W/m.Math.K.

34. A heat shield comprising the thermal insulation material of claim 29.

Description

DRAWINGS

[0050] FIG. 1 shows the thermal conductivity as a function of temperature for flexible felt products according to prior art. These products, based on carbon fibers obtained from isotropic pitch, are sold by the company Kureha under the trademark Kreca™; the figure is copied from their website (see http://www.kurehacarbonproducts.com/kreca-fr.html).

[0051] FIGS. 2 and 3 are schematic representations of carbon fibers with binder. FIG. 2 represents a case were the binder does not wet the carbon fiber but accumulates on the location where two or more carbon fibers are in contact. FIG. 3 shows a case where the binder wets the whole carbon fiber.

[0052] FIG. 4 shows the thermal conductivity as a function of temperature for four thermal insulation materials as explained in the section “Examples”. Samples (a), (b), and (c) are either prior art or outside of the scope of the present invention, while sample (d) represents the invention.

[0053] FIG. 5 shows a curve similar to FIG. 4. Samples (d), (e), (f), and (g) are according to the invention, while sample (b) is outside of the scope of the invention.

[0054] FIG. 6 shows three scanning electron micrographs of a thermal insulation material according to the invention at three different magnification levels.

[0055] FIG. 7 shows measured values for the zeta-potential of two types of fibers, namely fiber 1 according to prior art, and fiber 2 according to the invention.

DESCRIPTION

[0056] In a first step the raw materials are provided. The rare materials are carbon fibers and binder. According to an essential feature of the present invention, carbon fibers must be used as short filaments, with a length not exceeding about 2,000 μm and advantageously comprised between about 100 μm and about 2,000 μm, and preferably between about 200 μm and about 1,500 μm. The binder is a chemical compound which upon heat treatment up to 700° C. or more under non oxidizing atmosphere leaves a carbon residue which has at least 10% of the initial mass of the binder, and preferably at least 20%; this percent value is also known as the “carbon yield” of the binder under pyrolysis conditions. The binder is contained in a liquid phase, and can be used as a solution in an appropriate solvent, and/or in suspension, or as a slurry.

[0057] In this first step, either carbon fibers with embedded carbon black particles are provided directly as short carbon filaments with an average length not exceeding about 2,000 μm, or carbon fibers are provided and cut and/or milled down to an average length not exceeding about 2,000 μm.

[0058] In a second step, a slurry of said carbon fibers and said binder is prepared. Typically, the short carbon fibers are introduced in a liquid phase made from an appropriate liquid and a determined amount of binder dissolved or dispersed in said liquid. A thorough mixing and/or stirring of resulting liquid phase creates a slurry, composed of carbon fibers dispersed in said liquid phase.

[0059] In a third step, known as slurry casting, the slurry is poured into a mold made with a porous material, allowing the liquid to pass, but retaining the fibers. Leaving sufficient time, layers of fibers are deposited in the mold to form a so-called “wet green shape” or “wet green body”, which is basically a wet mass of fibers with almost no cohesion.

[0060] The fourth step is a drying and solidification step. The wet green shape is dried so that the remaining liquid can be evaporated, but not the binder which precipitates (“settles”) on the surface of the fibers. Once the drying is completed, the dried shape (called here “dried green shape” or “dried green body”) is brought to a temperature high enough to create a physical and/or chemical transformation of the binder molecules which are transformed into a solid, cross-linked network, connecting the carbon fibers together. If the carbonizable binder is a thermosetting binder, such as a water-soluble phenolic resin, this transformation is typically carried out at a temperature in the order of 150° C. to 250° C., depending on the properties of the binder.

[0061] At this stage, the green body has turned into a solid, with a set structure in which the spatial arrangement of the fiber and the connecting binding network is established and cannot be changed. This solid body can be easily handled.

[0062] The fifth step is a high temperature thermal treatment. The solid obtained from drying and solidifying the green body is heat treated under non-oxidizing conditions to a temperature of at least 700° C., in order to transform the binding network resulting from the solidification step into a network made exclusively of carbon; during this transformation the solidified binder undergoes a pyrolysis.

[0063] The fourth and fifth step can be carried out as a continuous heating process or in two steps (one for drying and thermosetting, another for carbonization); the latter is preferred.

[0064] Depending on the intended use, the heat treatment can be pursued to temperatures up to 2,400° C. in order to promote a thermally stable structure for end users which are going to use the insulation material for very high temperature processes.

[0065] Considering the thermal opacity of the final heat insulation material, the spatial distribution of the fiber-binder network will be a determining factor. For instance, a process leading to dense clusters (local areas, characterized by a high fiber volume content) separated by low density areas will be less opaque than a product with a perfectly even distribution of the fibers; the former process will therefore be less favorable to the obtention of a heat insulation material with good thermal insulation performance than the latter.

[0066] The present inventors have found that said spatial distribution is determined by several factors among which: the binder-fiber interaction while the binder is still dissolved and/or suspended in the liquid phase, the viscosity of the slurry, the operating conditions of the slurry casting operation, and the drying conditions used for the green body.

[0067] Furthermore, the present inventors have found that when carbon fibers containing carbon black particles, such as the ones described in the patent CN 106,245,226, are used with a binder, these fibers promote a drastically different fiber-binder interaction, while the binder is still dissolved and/or suspended in the liquid phase, from what is commonly observed with fibers that do not contain carbon black. This difference can be detected for instance by measuring the wetting angle between the fiber and the solvent+binder solution. This different interaction promotes in turn a different behavior of the slurry during the slurry casting operation. Clusters of fibers are almost entirely avoided and the consistency of the wet green body much improved, with less variations between the fiber volume fraction between the bottom layers and the bottom layers of the wet green body for instance.

[0068] It has also been observed that during the drying step following the casting, when the binder molecules are forced to precipitate on the fiber surface, there is less coagulation of the binder residue; coagulation of the binder is commonly observed when the binder has a limited affinity with the surface of the fiber. While the inventors do not wish to be bound by this theory, they believe that this phenomenon and its negative consequences on the thermal insulation performance can be better understood through the schematic representations on FIG. 2.

[0069] FIG. 2 schematically how a wet green body made with carbon fibers having a poor wettability for binder. During the drying step of the wet green body as the concentration of the binder increases due to solvent evaporation, a poor wetting of the fiber by the binder molecule will promote the regrouping and accumulation of binder molecules on the locations where two or more carbon fibers are in contact, which is the most favorable mechanism to minimize the total energy of the system. The coagulation of the binder at the intersection of carbon fibers provides, once the heat treatment is completed, larger bridges between the fibers, promoting a better thermal communication from one fiber to another, thereby increasing thermal conductivity. This is detrimental to the thermal insulation performance of the resulting body.

[0070] FIG. 3 schematically shows a wet green body made with carbon fibers having a good wettability for binder. During the drying step of the wet green body as the concentration of the binder increases due to solvent evaporation, a good wetting of the fiber by the binder molecule will promote the distribution of the binder molecules over the entire surface of the fibers.

[0071] As mentioned above, according to a second essential feature of the present invention, the carbon fibers used in the present invention must contain carbon black particles. Carbon black as such is known to a person skilled in the art, and its origin, morphology and structure will not be explained here further, except to recall that carbon black is an industrial product composed of carbon particles which are nanosized particles with a diameter range of about 10 nm to a few hundreds of nanometres. In the framework of the present invention, a mean size comprised between about 10 nm and about 100 nm is preferred, and still more preferred is a mean size of between about 20 nm and about 70 nm.

[0072] Such carbon black nanoparticles can be incorporated into polymer fibers which are precursors for carbon fibers. Such precursor fibers can be Rayon fibers, for example. More precisely, the carbon black particles are incorporated into the liquid mass from which polymer fibers are produced; these polymer fibers are then pyrolyzed into carbon fibers. During pyrolysis, the carbon black particles remain virtually unchanged. The resulting carbon fibers therefore comprise nanosize carbon black particles. These carbon fibers can be used in the process according to the present invention.

[0073] The thermal insulation material obtainable from the process according to the invention are typically rigid materials. They can have the form of board or plate, or can be of three-dimensional shape. Three dimensional shapes can be obtained at the green body stage. Such three-dimensional shapes can be plates, curved plates, hollow shapes, tubular shapes and so on, as may be needed. More generally, the thermal insulation material according to the invention can be used for the manufacture of heat shields, which can consist in said thermal insulation material according to the invention, or which can comprise said thermal insulation material according to the invention.

[0074] FIG. 6 shows a typical microstructure of a typical thermal insulation material according to the invention. The figure in the left shows the general morphology of the material; the length of the white bar is 20 mm. The figure in the center shows the mesoscopic scale, the length of the white bar being 40 μm. The individual carbon fibers can be distinguished, as well as the void between adjacent fibers. The figure on the right shows the nanoscopic structure inside the carbon fiber, the length of the black bar being 10 nm.

[0075] The thermal insulation material according to the invention are useful at any temperature, but are particularly useful when used at a temperature above 1,800° C., and in particular at temperature above 2,000° C., and possibly even as high as 2,300° C., in non-oxidizing atmosphere. Due to the very weak outgassing of the thermal insulation material, in particular when said third heat treatment step has been carried out, an advantageous use is a use with equipment for the manufacture of high-purity materials such as semi-conductors, glasses, ceramic. Said manufacturing processes for high-purity semiconductors include crystal growing.

[0076] The thermal insulation material according to the invention is made entirely from carbonaceous materials and has preferably been heat treated at high temperature (preferably at least 1,500° C.). This avoids the contamination with hetero atoms (such as oxygen or nitrogen) of goods processed in said equipment for the manufacture of materials in which said thermal insulation material is eventually used. Absence of contamination with hetero atoms is particularly important for high purity materials such as semiconductors, ceramics and glasses for specific applications in which the presence of impurities causes specific, unwanted physical or chemical effects. In the case of semiconductors, these effects can be related to doping; in the case of glasses they can be related to optical absorption.

[0077] In a particularly advantageous embodiment of the invention, said thermal insulation material has a thermal conductivity at 1,000° C. that does not exceed 0.25 W/m.Math.K, preferably does not exceed 0.23 W/m.Math.K, and still more preferably does not exceed 0.20 W/m.Math.K, and/or said thermal insulation material has a thermal conductivity at 1,700° C. that does not exceed 0.50 W/m.Math.K, preferably does not exceed 0.45 W/m.Math.K, and still more preferably does not exceed 0.40 W/m.Math.K. Such a thermal insulation material is particularly useful for a use with equipment for the manufacture of high purity materials, as mentioned above, in particular for making high purity semiconductors; it can be used for example with crystal growing equipment.

[0078] The use of the thermal insulation material according to the invention in oxidizing atmosphere is possible, too, but not recommended above 350° C.

EXAMPLES

[0079] In a first series of experiments, four samples labelled (a), (b), (c) and (d) were produced, as follows:

[0080] Sample (a): Carbon felt according to the state of the art

[0081] Carbon fibers ex-Rayon were cut to 60 mm average length, needled into a felt 11 mm thick, and heat-treated in a non-oxidizing atmosphere up to 2,300° C. The result is a graphitized carbon fiber felt, with a thickness of 10 mm and an apparent density of 0.09 gr/cc.

[0082] Sample (b): Rigid insulation board outside of the invention

[0083] Carbon fibers ex-Rayon were cut at a 700 μm average length, processed in a phenolic resin/water solution (12% (w/w) of phenolic resin in water 88% (w/w)), cast, dried, solidified, and heat treated at 2,300° C. The final product has an apparent density of 0.16 gr/cc.

[0084] Sample (c): Carbon felt outside of the invention

[0085] Same as sample (a) but for the fact that the Rayon fibers are containing 5% (w/w) of carbon black particles added into the Rayon melt prior to the spinning (extrusion) of the Rayon fibers. The final carbon felt product has a thickness of 10 mm and an apparent density of 0.09 gr/cc.

[0086] Sample (d): Rigid insulation board according to the invention

[0087] The process is the same as for sample (b) except that the ex-Rayon carbon fibers have been replaced by carbon fibers obtained from Rayon fibers produced from a Rayon melt containing 5% (w/w) of carbon black particles. The final product has an apparent density of 0.14 gr/cc.

[0088] As can be seen from FIG. 4 the material of the invention (d) exhibits a favorable thermal insulation performance at both ends of the temperature range.

[0089] The result on the low temperature range (from room temperature up to 1,000° C.) was expected as a result of the low thermal conductivity of the carbon fibers with the added carbon black particles, as claimed by the patent documents RU 2,535,797 and CN 106,245,226 mentioned above, and of the lower density (compared to classic rigid board) made possible by the better wetting of the carbon fiber.

[0090] The result on the high temperature range is unexpected, and is the consequence of the invention, which promotes a structure which has a higher thermal opacity and a much improved thermal insulation performance which is detectable at temperatures of the order of 1,700° C. and above.

[0091] In a second series of experiments, three other samples of rigid thermal insulation board according to the invention, labelled (e), (f), and (g), were produced, as follows. All of them were based on carbon fibers manufactured from Rayon fibers containing carbon black particles.

[0092] Sample (e): Same as sample (d), wherein the phenolic resin 12%/water 88% solution has been replaced by a sugar 45%/water 55% solution to serve as a binder.

[0093] Sample (f): Same as sample (d), where the carbon black content in the rayon fibers is 3.5% (w/w) instead of 5% (w/w).

[0094] Sample (g): Same as sample (d), where the carbon black content in the rayon fibers is 10% (w/w) instead of 5% (w/w).

[0095] The results are shown on FIG. 5, which also compares these results with those obtained for rigid thermal insulation board obtained from the first series of experiments (samples (b) and (d)).

[0096] Table 1 summarizes all the samples that have been elaborated as examples.

TABLE-US-00001 TABLE 1 Sample Process Comment (a) Ex Rayon carbon fiber, long fiber, felt structure Prior art (b) Ex Rayon carbon fiber, short fiber + phenolic Outside of binder the invention (c) Ex Rayon carbon fiber + carbon black, felt Outside of structure the invention (d) Ex Rayon carbon fiber + carbon black particles According to (5%), short fiber + phenolic binder the invention (e) Ex Rayon carbon fiber + carbon black particles According to (5%), short fiber + “sugar binder” the invention (f) Ex Rayon carbon fiber + carbon black particles According to (3.5%), short fiber + phenolic binder the invention (g) Ex Rayon carbon fiber + carbon black particles According to (10%), short fiber + phenolic binder the invention

[0097] In another series of experiments, the zeta potential of two types of fibers was measured using a Malvern Zetasizer™ in a slurry prepared with pure water as the liquid phase; the electrical conductivity of the slurries was 0.05 mS/cm.

[0098] Fiber 1 contains no carbon black particles; this is a fiber as used in a prior art processes.

[0099] Fiber 2 contains carbon black particles; this is a fiber as used in a process according to the present invention.

[0100] The result is shown on FIG. 7. It can be seen that both fibers are negatively charged. The zeta potential of fibers containing carbon black particles is more negative (i.e. is negative and has a higher absolute value) than that of fibers containing no carbon black particles.

[0101] The zeta potential measurement is rather approximate and depends on the ionic strength of the medium. The addition of salts will cause a slight reduction of the absolute value of the zeta potential.

[0102] This result comes in support of the observation that the fibers containing carbon black particles are easier to disperse in the water-borne slurry than fibers containing no carbon black particles. The lower zeta potential tends to favor a better distribution and orientation of the fibers in the thermal insulation material.