Heat insulating material suitable for use at high temperatures, and method for manufacturing the above material

Abstract

A process for making a carbon-based insulating material suitable for use at high temperatures and a method for making said material, suitable for use at temperatures above 1,500°C, comprising the steps of providing carbon fibers having embedded therein carbon black particles, cutting or grinding the carbon fibers to obtain short carbon filaments, preparing a slurry by introducing the short carbon filaments into a liquid phase containing a binder capable of forming a carbon residue upon pyrolysis under non-oxidizing conditions, pouring 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 the insulating material.

Description

Technical Field

[0001] The present invention relates to materials science, and more specifically to heat insulators. In particular, the present invention relates to a novel method for manufacturing a heat insulator using carbon fibers that is suitable for use at temperatures above 1,500 °C and has a very low thermal conductivity. The present invention also relates to the heat insulator itself.

Background Art

[0002] Heat insulators formed from carbon fibers are well known and are widely used to provide heat insulation in industrial systems operating at high temperatures, particularly systems operating at temperatures above 1,000 °C (one thousand degrees Celsius). Their success is due to their unique stability under high temperature conditions in an oxygen-free atmosphere, their low thermal conductivity, abundance, and relatively low cost.

[0003] Carbon fibers are known as such and are manufactured in large quantities worldwide. Their industrial production processes are widely documented (see, for example, "Carbon Fiber: manufacture and application" by Vincent Kelly, Elsevier Science and Technology, 2004). It is typically obtained by pyrolysis of organic precursor fibers. The above-mentioned precursor fibers can be derived from various natural sources or can be synthetic polymeric fibers. A typical natural precursor fiber is cotton fiber. Typical synthetic polymeric precursor fibers are rayon or polyacrylonitrile (PAN), etc. Pitch can also be used as a precursor for carbon fibers. Polyethylene and polystyrene have also been used (see International Publication No. WO 2017 / 167941 assigned to Total Research & Technology Feluy). The fiber manufacturing method needs to be adapted for each type of precursor. Most of these processes include a step of converting a liquid precursor into precursor fibers by means such as extrusion or melt spinning. Then, those precursor fibers are converted into carbon fibers by undergoing a somewhat advanced pyrolysis (carbonization) treatment consisting of heating the precursor fibers to at least 900 °C in an oxygen-free atmosphere. Higher heat treatment temperatures are often required to produce carbon fibers with high mechanical properties, and high mechanical properties at low density are the most prominent feature of carbon fibers and have brought about most of their industrial applications.

[0004] There are two types of commercially available thermal insulation materials formed from carbon fibers. One type can be described as "flexible" and consists of carbon fibers arranged by common textile techniques (weaving, needling, carding, and felting). They are typically manufactured as continuous webs characterized by a certain thickness and a certain width. They are generally used by attaching them to a solid support, thereby determining the shape.

[0005] The other type can be described as "rigid" and consists of carbon fibers bound by a carbonaceous binder. These materials are manufactured as rigid or semi-rigid parts, such as flat plates or cylinders, which can form self-supporting thermal shields and can be produced by mechanical means.

[0006] The flexible type is characterized by an apparent density in the range of 0.05 - 0.15 g / cm 3 whereas the rigid type has a density in the range of 0.10 - 0.30 g / cm 3 Higher densities can be easily achieved, but higher densities increase the thermal conductivity of the material and are rarely used as they are detrimental to the intended use.

[0007] The main function of the thermal insulation material is to prevent heat loss in industrial processes or to protect a specific structure or component from high heat. These are better achieved goals when using materials that inherently exhibit low thermal conductivity. Therefore, it is desirable to further improve the thermal insulation performance of these materials. In fact, these materials are being studied in detail to understand how their composition and manufacturing process conditions affect their thermal insulation properties, with the ultimate goal of producing solutions with excellent insulation performance.

[0008] A technical report from Oak Ridge National Laboratory published in 1973 by T.G. Godfrey and D.L. McElroy ("Thermal Conductivity of Oriented Fibrous Carbon Insulation from 300 to 1300 K in Nitrogen and Argon at one Atmosphere") established two important relationships. - The thermal conductivity of the constituent carbon fibers and the thermal conductivity of the insulation material are related by the following equation,

Equation

Number

[0009] Crucially, it is outlined by the same study that the above equation 2 established to explain the thermal conductivity of the insulating material as a function of temperature is only valid for temperatures lower than 1,300 K, which is the region where the heat transfer mechanism is dominated by conduction in the constituent materials (as established in equation 1). Above this temperature, the contribution of radiation becomes the dominant heat transfer mechanism. As a result, above 1,300 K, the thermal conductivity increases rapidly with the temperature of the material.

[0010] The drawbacks of insulating materials made of fibers with a low fiber volume fraction when used at very high temperatures are well known to those skilled in the art and can be deduced from the technical literature and all available commercial literature advertising the thermal properties of such materials as a function of temperature.

[0011] In these high-temperature ranges, the thermal conductivity strongly depends on the ability of the material to prevent efficient radiative mechanisms, and this property can be summarized as the "opacity" of the material. This property, which is essential for good thermal insulation performance at high temperatures (T > 1,300 K), depends less on the intrinsic thermal conductivity of the fibers and more on the shape and spatial distribution of the individual entities. This can be seen from Figure 1 showing the temperature-dependent thermal conductivity for a flexible felt based on carbon fibers provided by Kuraray and obtained from its isotropic pitch, and this flexible felt is sold under the trademark Kreca® (see http: / / www.kurehacarbonproducts.com / kreca-fr.html). The figure shows the thermal conductivity of two felts with densities of 0.13 g / cm 3 and 0.16 g / cm 3 .

[0012] According to the orientation provided by Equation 1, many carbon fiber-based insulating materials have been developed using carbon fibers with the lowest possible thermal conductivity. As an example, US 6,800,364 (assigned to UCAR Carbon Company Inc.) discloses a rigid insulating carbon material based on carbon fibers made from isotropic pitch, and this material is thought to provide a lower thermal conductivity than conventionally used rayon-based carbon fibers.

[0013] RU 2,535,797 C1 describes the use of polyacrylonitrile (PAN) fibers containing carbon black particles for producing thermal insulation materials. According to this patent, the addition of carbon black particles into the PAN fibers results in low-thermal-conductivity fibers and is advantageous for the production of high-performance carbon fiber felts for thermal insulation purposes.

[0014] CN106,245,226A (Yu Muhuo et al.) discloses a high-performance heat-insulating felt based on carbon fibers produced from a melt of cellulose containing the addition of carbon black particles. According to this patent, the carbon fibers produced by this method have a very low thermal conductivity, which is due to the fact that carbon black particles are embedded in the carbon fibers, and thus these carbon fibers are suitable for the production of carbon fiber-based felts with excellent heat-insulating performance.

[0015] While all inventions related above seek to achieve the purpose of manufacturing excellent insulating materials by using low-thermal-conductivity carbon fibers, none of them address the specific problem of heat-insulating performance at temperatures above 1,300 K, especially above 1,500 °C, where radiation becomes the dominant heat transfer mechanism.

[0016] The problem addressed by the present invention is to provide a new heat-insulating material having improved heat-insulating characteristics at particularly high temperatures above about 1,000 °C, especially above 1,500 °C.

Summary of the Invention

Problems to be Solved by the Invention

[0017] According to the present invention, the problem is solved by a process in which carbon fibers embedded with carbon black particles are bonded by a pyrolyzable binder, and then the pyrolyzable binder is pyrolyzed to form carbon residues on the fibers. The inventors have observed that on carbon fibers embedded with carbon black particles, the binder diffuses to cover substantially all of the fiber surface and does not accumulate only at the contact points between two fibers. As a result, after pyrolysis, the carbon fibers are coated with a layer of carbon residues having the effect of reducing their thermal emissivity, thereby reducing the radiative heat transfer between the fibers.

[0018] According to the first essential feature of the present invention, the carbon fiber must contain carbon black particles. The mass fraction of the carbon black particles in the carbon fiber is preferably composed between 1% and 50%, more preferably composed between 3% and 40%, and even more preferably composed between 10% and 35%.

[0019] According to the second essential feature of the present invention, the carbon fiber must be short in order to facilitate its coating by binder molecules and avoid the accumulation of the binder at the contact points. Regarding short carbon fibers, the carbon residue obtained after the drying and pyrolysis of the binder covers most of the fiber length and does not accumulate only on the contact points between two fibers. Furthermore, short fibers are less chaotic than long fibers, thereby obtaining a denser green body, which, after appropriate heat treatment, results in a rigid thermal insulation material with a higher opacity at a very high temperature.

[0020] By the process according to the present invention, a rigid or semi-rigid thermal insulation material can generally be obtained as a molded part such as a board, cylinder, or bucket. The above material can be mechanically produced.

[0021] The thermal opacity (i.e., the heat insulation property) of the material of this invention at a very high temperature, especially above 1,500 °C or above 2,000 °C, is better than that generally achievable using carbon fiber-based felts or rigid boards according to the prior art.

[0022] The first object of the present invention is a process for producing a thermal insulation material containing carbon fibers, the process comprising - preparing carbon fibers embedded with carbon black particles, - cutting and / or grinding the carbon fibers to obtain short carbon filaments with an average length not exceeding about 2,000 μm, - Introducing the short carbon filaments into a liquid phase containing a binder capable of forming a carbon residue corresponding to 10% or more of the initial mass of the binder when heat-treated at a temperature of at least 700 °C under non-oxidizing conditions, to prepare a slurry; - Pouring the slurry into a mold capable of separating the slurry into a wet green body and a liquid phase; - Heat-treating the wet green body in at least two steps, namely a drying step capable of drying and solidifying the binder, and a pyrolysis step carried out at a minimum temperature of 700 °C under non-oxidizing conditions and capable of converting the binder into a carbon residue; Comprising.

[0023] Advantageously, the carbon fibers have an average diameter not exceeding 20 μm, preferably not exceeding 15 μm, and more preferably an average diameter composed between 1 μm and 10 μm.

[0024] The liquid phase may contain water.

[0025] During slurry casting, the mold acts as a filter and retains the wet carbon fibers.

[0026] The binder should have a carbon yield of at least 10%, preferably at least 20%, and more preferably at least 30%. The binder may contain auxiliaries such as phenolic resins, sugars, and / or starches.

[0027] The carbon fibers embedded with carbon particles are as follows: - Preparing a polymeric carbonizable fiber, so-called "precursor fiber", embedded with carbon black particles; - Subjecting the precursor fiber to a first heat treatment at a temperature of at least 700 °C, preferably at least 800 °C, more preferably at least 900 °C, to obtain carbon fibers containing embedded carbon black particles; Can be obtained by a process comprising.

[0028] In an embodiment of the above process for producing a heat insulating material containing carbon fibers, the drying and heat treatment steps include a drying step capable of drying and solidifying the binder, and a pyrolysis step carried out at a minimum temperature of 700 °C under non-oxidizing conditions, which can convert the binder into carbon residue. The temperature is preferably at least 800 °C, more preferably at least 900 °C.

[0029] After the pyrolysis step, heat treatment can be continued at a temperature above 1,500 °C, preferably above 1,800 °C, more preferably above 2,000 °C under non-oxidizing conditions.

[0030] The carbon filaments can have an average length composed between about 100 μm and about 2,000 μm, preferably between about 200 μm and about 1,500 μm.

[0031] A second object of the present invention is a heat insulating material obtainable from the process according to the present invention. In particular, the heat insulating material can be manufactured as a rigid board or a rigid three-dimensional shape.

[0032] The material according to the present invention typically has a thermal conductivity at 1,000 °C not exceeding 0.25 W / m.K, preferably not exceeding 0.23 W / m.K, more preferably not exceeding 0.20 W / m.K, and / or a thermal conductivity at 1,700 °C not exceeding 0.50 W / m.K, preferably not exceeding 0.45 W / m.K, more preferably not exceeding 0.40 W / m.K.

[0033] A third object of the present invention is a heat shield made of or containing the heat insulating material according to the present invention.

[0034] Yet another object of the present invention is the use of the heat insulating material according to the present invention, or the heat shield according to the present invention, at a temperature above 1,300 °C, preferably above 1,500 °C, more preferably above 1,800 °C, particularly above 2,000 °C.

[0035] In particular, the above materials or thermal shields can be used together with equipment for the production of materials such as semiconductors, glass, ceramics, etc., and in particular, together with equipment for the production of high-purity semiconductors, including, for example, crystal growth equipment.

Brief Description of the Drawings

[0036]

Figure 1

Figure 2

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Figure 7

Modes for Carrying Out the Invention

[0037] In the first stage, raw materials are prepared. The raw materials are carbon fibers and a binder. According to the essential features of the present invention, the carbon fibers must be used as short filaments having a length not exceeding about 2,000 μm, preferably composed between about 100 μm and about 2,000 μm, and preferably composed between about 200 μm and about 1,500 μm. The binder is a compound that leaves a carbon residue of at least 10%, preferably at least 20% of the initial mass of the binder when heat-treated up to 700 °C or higher under a non-oxidizing atmosphere. This percentage value is also known as the "carbon yield" of the binder under pyrolysis conditions. The binder can be contained in a liquid phase and used as a solution of a suitable solvent and / or suspension, or as a slurry.

[0038] In this first stage, carbon fibers embedded with carbon black particles are directly prepared as short carbon filaments having an average length not exceeding about 2,000 μm, or the carbon fibers are prepared and cut and / or ground to an average length not exceeding about 2,000 μm.

[0039] In the second stage, a slurry of the above carbon fibers and the above binder is prepared. Typically, the short carbon fibers are introduced into a liquid phase made from a suitable liquid and a determined amount of binder dissolved or dispersed in the above liquid. By sufficiently mixing and / or stirring the resulting liquid phase, a slurry composed of carbon fibers dispersed in the above liquid phase is produced.

[0040] In a third stage known as slurry casting, the slurry is poured into a mold made of a porous material, and the liquid can pass through, but the fibers are retained. After a sufficient time, a layer of fibers is deposited in the mold to form a so-called "wet green shape" or "wet green body", which is a cluster of wet fibers with basically little aggregation.

[0041] The fourth stage is the drying and solidification stage. The wet green shape is dried such that the remaining liquid can evaporate, but the binder that precipitates (''settles'') on the surface of the fibers does not evaporate. Once drying is complete, the dried shape (referred to herein as the ''dry green shape'' or ''dry green body'') is brought to a temperature high enough to produce a physical and / or chemical transformation of the binder molecules, and the binder molecules are converted into a solid cross-linked network that connects the carbon fibers. When the carbonizable binder is a thermosetting binder such as a water-soluble phenolic resin, this transformation is typically carried out at a temperature on the order of 150 °C to 250 °C, depending on the properties of the binder.

[0042] At this stage, the green body has turned into a solid, with a fixed structure in which the spatial configuration of the fibers and the connecting bond network are established and cannot be changed. This solid can be easily handled.

[0043] The fifth stage is high-temperature heat treatment. The solid obtained from the stage of drying and solidifying the green body is heat-treated at a temperature of at least 700 °C under non-oxidizing conditions to convert the bond network obtained from the solidification stage into a network made only of carbon, and the binder solidified during this conversion is pyrolyzed.

[0044] The fourth and fifth stages can be carried out as a continuous heat treatment or in two stages (one for drying and thermosetting and the other for carbonization), with the latter being preferred.

[0045] Depending on the intended use, the heat treatment can be advanced to a temperature of up to 2400 °C to improve the thermal stability structure for end users who will use insulating materials for very high-temperature processes.

[0046] Considering the thermal opacity of the final insulation material, the spatial distribution of the fiber-binder network is a determining factor. For example, a process leading to densely packed clusters (local regions characterized by a high fiber volume fraction) separated by low-density regions results in a lower opacity than a product with a completely uniform distribution of fibers. Thus, the former process is less favorable than the latter for obtaining an insulation material with good thermal insulation performance.

[0047] The inventors have found that the above spatial distribution is determined by several factors, among which are the binder-fiber interactions 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.

[0048] Furthermore, the inventors have found that when carbon fibers containing carbon black particles, such as those described in Patent CN106,245,226, are used together with a binder, these fibers promote fiber-binder interactions while the binder is still dissolved and / or suspended in the liquid phase, which are significantly different from those normally observed with fibers not containing carbon black. This difference can be detected, for example, by measuring the wetting angle between the fiber and the solvent + binder solution. Consequently, this different interaction promotes different behaviors of the slurry during the slurry casting operation. Fiber clustering is almost completely avoided, and the coherence of the wet green body is much improved. For example, the variation in fiber volume fraction between the bottom layers of the wet green body is less.

[0049] During the drying stage after casting, it was also observed that there is less solidification of the binder residue when depositing the binder molecules on the fiber surface. The solidification of the binder is usually observed when the affinity of the binder for the surface of the fiber is limited. Although the inventors do not wish to be bound by this theory, they are convinced that this phenomenon and the negative results regarding its thermal insulation performance can be better understood by the schematic diagram of Figure 2.

[0050] Figure 2 schematically shows a wet green body made of carbon fibers having poor wettability with respect to the binder. During the stage of drying the wet green body while the concentration of the binder increases due to solvent evaporation, the poor wetting of the fibers by the binder molecules promotes the regrouping and accumulation of the binder molecules at locations where two or more carbon fibers are in contact. This is the most favorable mechanism for minimizing the total energy of the system. Once the heat treatment is completed, the solidification of the binder at the intersections of the carbon fibers provides a larger bridge between the fibers, promoting better heat transfer from one fiber to another, thereby increasing the thermal conductivity. This is detrimental to the heat insulation performance of the resulting body.

[0051] Figure 3 schematically shows a wet green body made of carbon fibers having good wettability with respect to the binder. During the stage of drying the wet green body while the concentration of the binder increases due to solvent evaporation, the good wetting of the fibers by the binder molecules promotes the distribution of the binder molecules over the entire surface of the fibers.

[0052] As mentioned above, according to the second essential feature of the present invention, the carbon fibers used in the present invention must contain carbon black particles. Such carbon black is known to those skilled in the art, and except for recalling that carbon black is an industrial product composed of carbon particles which are nano-sized particles having a diameter range of about 10 nm to several hundred nanometers, its origin, morphology, and structure are not further described herein. In the framework of the present invention, an average size composed between about 10 nm and about 100 nm is preferred, and an average size between about 20 nm and about 70 nm is more preferred.

[0053] Such carbon black nanoparticles can be incorporated into polymer fibers that are precursors of carbon fibers. Such precursor fibers can be, for example, rayon fibers. More precisely, the carbon black particles are incorporated into the liquid mass from which the polymer fibers are produced, and then these polymer fibers are pyrolyzed into carbon fibers. During pyrolysis, the carbon black particles remain substantially unchanged. Thus, the resulting carbon fibers contain nano-sized carbon black particles. These carbon fibers can be used in the process according to the present invention.

[0054] The heat insulating materials obtainable from the process according to the present invention are typically rigid materials. They can have the form of boards or plates, or can be three-dimensional in shape. The three-dimensional shape can be obtained at the green body stage. Such three-dimensional shapes can be, as required, plates, curved plates, hollow shapes, and tubular shapes, etc. More generally, the heat insulating materials according to the present invention can be used for the production of heat shields, which can consist of or include the above heat insulating materials according to the present invention.

[0055] Figure 6 shows the typical microstructure of a typical heat insulating material according to the present invention. The left figure shows the overall morphology of the material, and the length of the white bar is 20 mm. The middle figure shows the mesoscopic scale, and the length of the white bar is 40 μm. Individual carbon fibers as well as the voids between adjacent fibers can be identified. The right figure shows the nanoscopic structure inside the carbon fiber, and the length of the black bar is 10 nm.

[0056] The heat insulating materials according to the present invention are useful at any temperature, but are particularly useful when used in a non-oxidizing atmosphere at temperatures above 1,800 °C, especially above 2,000 °C, and in some cases even up to 2,300 °C. Due to the very weak gas evolution of the heat insulating materials, especially when the above third heat treatment step is carried out, an advantageous use is the use in equipment for the production of high-purity materials such as semiconductors, glass, ceramics, etc. The above production process for high-purity semiconductors includes crystal growth.

[0057] The heat insulating material according to the present invention is made entirely of a carbonaceous material and is preferably heat-treated at a high temperature (preferably at least 1,500 °C). Thereby, contamination by heteroatoms (such as oxygen or nitrogen) of an article processed in the equipment for manufacturing the material in which the heat insulating material is finally used is avoided. The absence of contamination by heteroatoms is particularly important for high-purity materials such as semiconductors, ceramics, and glass for certain applications where the presence of impurities causes certain undesirable physical or chemical effects. In the case of semiconductors, these effects can relate to doping, and in the case of glass, they can relate to light absorption.

[0058] In a particularly advantageous embodiment of the present invention, the heat insulating material has a thermal conductivity at 1,000 °C not exceeding 0.25 W / m·K, preferably not exceeding 0.23 W / m·K, more preferably not exceeding 0.20 W / m·K, and / or the heat insulating material has a thermal conductivity at 1,700 °C not exceeding 0.50 W / m·K, preferably not exceeding 0.45 W / m·K, more preferably not exceeding 0.40 W / m·K. Such a heat insulating material is particularly useful for use in equipment for manufacturing high-purity materials as mentioned above, especially for manufacturing high-purity semiconductors, which can be, for example, use in crystal growth equipment.

[0059] Use of the heat insulating material according to the present invention in an oxidizing atmosphere is possible, but it is not recommended to exceed 350 °C.

Examples

[0060] In a first series of experiments, four samples labeled (a), (b), (c), and (d) were prepared as follows.

[0061] Sample (a): Carbon felt according to the state of the art Rayon-derived carbon fibers were cut to an average length of 60 mm, 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 having a thickness of 10 mm and an apparent density of 0.09 gr / cc.

[0062] Sample (b): A rigid insulating board outside the present invention Rayon-derived carbon fibers were cut at an average length of 700 μm, treated in a phenol resin / water solution (12% (w / w) phenol resin in 88% (w / w) water), cast, dried, solidified, and heat-treated at 2,300 °C. The final product has an apparent density of 0.16 gr / cc.

[0063] Sample (c): A carbon felt outside the present invention Same as sample (a), except that the rayon fibers contain 5% (w / w) carbon black particles added to 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.

[0064] Sample (d): A rigid insulating board according to the present invention The process is the same as that for sample (b), except that the rayon-derived carbon fibers are replaced with carbon fibers obtained from rayon fibers produced from a rayon melt containing 5% (w / w) carbon black particles. The final product has an apparent density of 0.14 gr / cc.

[0065] As can be seen from Figure 4, the material (d) of the present invention exhibits advantageous heat insulation performance at both ends of the temperature range.

[0066] The results for the low temperature range (from room temperature up to 1,000 °C) are expected as claimed by the above - mentioned patent documents RU2,535,797 and CN106,245,226, as a result of the low thermal conductivity of carbon fibers with carbon black particles added, and a lower density (compared to conventional rigid boards) made possible by better wetting of the carbon fibers.

[0067] The results for the high temperature range were not expected and are a result of the present invention, which promotes structures with higher thermal opacity and much improved heat insulation performance detectable at temperatures of 1,700 °C and above.

[0068] In a second series of experiments, three other samples of the rigid thermal insulation board according to the present invention, labeled (e), (f), and (g), were prepared as follows. All of them were based on carbon fibers manufactured from rayon fibers containing carbon black particles.

[0069] Sample (e): It was the same as sample (d), where a solution of 12% phenol resin / 88% water was replaced with a solution of 45% sugar / 55% water to act as a binder.

[0070] Sample (f): It was the same as sample (d), where the carbon black content in the rayon fibers was 3.5% (w / w) instead of 5% (w / w).

[0071] Sample (g): It was the same as sample (d), where the carbon black content in the rayon fibers was 10% (w / w) instead of 5% (w / w).

[0072] The results are shown in Figure 5. In Figure 5, these results are also compared with those obtained for the rigid thermal insulation boards from the first series of experiments (samples (b) and (d)).

[0073] Table 1 summarizes all the samples described in detail as examples. Table 1

Table 1

[0074] 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 conductivity of the slurry was 0.05 mS / cm.

[0075] Fiber 1 does not contain carbon black particles and is a fiber such as those used in prior art processes.

[0076] Fiber 2 does not contain carbon black particles and is a fiber such as those used in the process according to the present invention.

[0077] The results are shown in Figure 7. It can be seen that both fibers are negatively charged. The zeta potential of the fiber containing carbon black particles is more negative (i.e., negative and having a larger absolute value) than that of the fiber not containing carbon black particles.

[0078] The measurement of the zeta potential is rather approximate and depends on the ionic strength of the medium. When salt is added, the absolute value of the zeta potential is slightly reduced.

[0079] This result supports the observation that fibers containing carbon black particles disperse more readily in an aqueous slurry than fibers not containing carbon black particles. A lower zeta potential tends to be favorable for a better distribution and orientation of the fibers in the thermal insulation material.

Claims

1. A method for manufacturing a heat insulating material containing carbon fibers, comprising: - preparing carbon fibers embedded with carbon black particles; - cutting or pulverizing the carbon fibers to obtain short carbon filaments with an average length not exceeding 2,000 μm; - preparing a slurry by introducing the short carbon filaments into a liquid phase containing the binder that can form a carbon residue corresponding to 10% or more of the initial mass of the binder when heat-treated at a temperature of at least 700°C under non-oxidizing conditions; - pouring the slurry into a mold capable of separating the slurry into a wet green body and a liquid phase; - drying and heat-treating the wet green body to obtain a heat insulating material. A method for manufacturing a heat insulating material containing carbon fibers, comprising the above steps.

2. The drying and heat-treating step includes a drying step capable of drying and solidifying the binder, and a pyrolysis step performed at a minimum temperature of 700°C under non-oxidizing conditions to convert the binder into a carbon residue. The manufacturing method according to Claim 1.

3. The pyrolysis step is performed at a temperature of at least 800°C. The manufacturing method according to Claim 2.

4. The pyrolysis step is performed at a temperature of at least 900°C. The manufacturing method according to Claim 3.

5. After the pyrolysis step, heat treatment is continued at a temperature exceeding 1,500°C under non-oxidizing conditions. The manufacturing method according to any one of Claims 2 to 4.

6. After the pyrolysis step, heat treatment is continued at a temperature exceeding 1,800°C under non-oxidizing conditions. The manufacturing method according to Claim 5.

7. After the pyrolysis step, heat treatment is continued at a temperature exceeding 2,000°C under non-oxidizing conditions. The manufacturing method according to Claim 6.

8. The short carbon filaments have an average length composed between 100 μm and 2,000 μm. The manufacturing method according to any one of Claims 1 to 7.

9. The short carbon filaments have an average length composed between 200 μm and 1,500 μm. The manufacturing method according to Claim 8.

10. The carbon yield of the binder is at least 10%. The manufacturing method according to any one of Claims 1 to 9.

11. The carbon yield of the binder is at least 20%. The manufacturing method according to Claim 10.

12. The manufacturing method according to any one of claims 1 to 11, wherein the mass fraction of the carbon black particles in the carbon fiber is composed between 1% and 50%.

13. The manufacturing method according to claim 12, wherein the mass fraction of the carbon black particles in the carbon fiber is composed between 3% and 40%.

14. The manufacturing method according to claim 13, wherein the mass fraction of the carbon black particles in the carbon fiber is composed between 10% and 35%.

15. The manufacturing method according to any one of claims 1 to 14, wherein the binder contains an auxiliary agent selected from the group formed by a phenol resin, sugar, and starch.

16. The manufacturing method according to any one of claims 1 to 15, wherein the liquid phase contains water.

17. The manufacturing method according to any one of claims 1 to 16, wherein the carbon fiber has an average diameter not exceeding 20 μm.

18. The manufacturing method according to claim 17, wherein the carbon fiber has an average diameter not exceeding 15 μm.

19. The manufacturing method according to claim 18, wherein the carbon fiber has an average diameter composed between 1 μm and 10 μm.

20. Use of a heat shield that can be obtained from the manufacturing method according to any one of claims 1 to 19 at a temperature exceeding 1,300 °C, or consisting of or including a heat insulating material that can be obtained from the manufacturing method according to any one of claims 1 to 19.

21. Use of the heat insulating material or heat shield according to claim 20 at a temperature exceeding 1,500 °C.

22. Use of the heat insulating material or heat shield according to claim 21 at a temperature exceeding 1,800 °C.

23. Use of the heat insulating material or heat shield according to claim 22 at a temperature exceeding 2,000 °C.

24. The use of the heat shield according to any one of claims 20 to 23, wherein the heat insulating material or heat shield is used together with equipment for the manufacture of at least one of a semiconductor, glass, or ceramic.

25. The use of the heat shield according to claim 24, wherein the heat insulating material or heat shield is used together with equipment for the manufacture of high-purity semiconductors, including crystal growth equipment.

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