Carbon fiber substrates, prepregs, porous structures and their manufacturing methods, preforms, fiber-reinforced resin molded bodies, sandwich structures and aircraft components

Abstract

The object of this invention is to obtain a carbon fiber substrate that achieves both high levels of heat resistance and processability, and to provide a prepreg or porous structure using the carbon fiber substrate that exhibits high heat resistance. This invention is a carbon fiber substrate comprising carbon fibers (A) and a resin composition (B) covering the surface of the carbon fibers (A), wherein the carbon fibers (A) are formed into a nonwoven fabric, and the tensile strength of the carbon fiber substrate after heating at 300°C for 5 minutes is at least 110% of its strength before heating.

Description

Carbon fiber substrates, prepregs, porous structures and their manufacturing methods, preforms, fiber-reinforced resin molded bodies, sandwich structures and aircraft components This invention relates to a carbon fiber substrate and a prepreg. Fiber-reinforced composite materials, which use carbon fiber or other reinforcing fiber substrates as reinforcements and impregnate their pores with matrix resins, are lightweight while exhibiting excellent mechanical properties such as strength and rigidity. Therefore, they are used in various fields including aerospace, automotive, railway vehicles, shipbuilding, civil engineering, electronic machinery, industrial machinery, and sporting goods. In particular, using carbon fiber nonwoven fabric in prepregs of carbon fiber substrates or expanding it to create porous structures offers advantages such as excellent mechanical properties and superior processability for complex shapes, including thin walls or three-dimensional forms. Furthermore, using nonwoven fabrics formed by horizontally dispersing and isotropically aligning reinforcing fibers in prepregs of reinforcing fiber substrates results in isodirectional mechanical properties, eliminating the need for lamination to compensate for weaker directions, as is the case with quasi-isotropic laminations. This allows for greater design freedom in laminated structures. This type of prepreg can be used as a single layer with thin walls, or as a preform formed by freely stacking multiple prepreg sheets for molding purposes. It can be molded into a void-free fiber-reinforced resin molded body by heating and pressurizing, or into a porous body by heating and expansion. Therefore, it has the advantage of excellent processability for complex shapes such as thin walls or three-dimensional shapes. Patent Document 1 shows a carbon fiber substrate with an adhesive and a fiber-reinforced composite material impregnated therein with a matrix resin. On the other hand, when developing these applications, the coexistence of processability and heat resistance remains a continuous challenge. In particular, the market demand for matrix resins is pushing for higher heat resistance, leading to higher temperatures and pressures in the impregnation process. This raises the issue of developing a carbon fiber substrate that can withstand such processes while maintaining excellent processability. Patent Documents 2 and 3 disclose carbon fibers coated with polyetherimide as a sizing agent, used to obtain fiber-reinforced resins with high heat resistance. [Prior Art Documents] [Patent Documents] Patent Document 1: International Publication No. 2010 / 013645; Patent Document 2: Japanese Patent Publication No. 02-002990; Patent Document 3: Japanese Patent Publication No. 2014-500912. [The problem that the invention aims to solve] Patent Document 1 does not disclose the challenges of further improving the heat resistance and formability of the carbon fiber substrate corresponding to the high temperature and high compression process associated with the use of high melting point matrix resin. Patent Documents 2 and 3 are technologies related to sizing agents and are insufficient for impregnation or coating in reinforcing fibers in non-woven fabric shapes. The object of this invention is to provide a carbon fiber substrate that combines processability and heat resistance. [Means for solving the problem] The present invention is a carbon fiber substrate comprising carbon fiber (A) and a resin composition (B) covering the surface of the carbon fiber (A), wherein the carbon fiber (A) forms a nonwoven fabric, and the tensile strength of the carbon fiber substrate after being heated at 300°C for 5 minutes is more than 110% of that before heating. In addition, the present invention is a prepreg made by impregnating a matrix resin (C) in a carbon fiber substrate derived from the present invention (first state sample). Furthermore, the present invention is a prepreg made by impregnating a matrix resin (C) into a substrate comprising carbon fiber (A) and an adhesive resin composition (D), wherein the carbon fiber (A) is a discontinuous fiber forming a nonwoven fabric, the glass transition temperature of the adhesive resin composition (D) is 120°C or higher and 450°C or lower, the melting point of the matrix resin (C) is 250°C or higher and 400°C or lower, and the adhesive resin composition (D) is present at the interface between the carbon fiber (A) and the matrix resin (C). The prepreg satisfies the following conditions (i) and (ii) (second state sample): (i) The minimum tensile strength is 100 MPa or higher. (ii) The thickness deviation at 400°C is 10% or lower. In addition, the present invention is a porous structure formed from the prepreg of the present invention. In addition, the present invention is a method for manufacturing a porous structure, which is a method for manufacturing the porous structure of the present invention, and includes: a first step of melting and expanding the matrix resin (C); and a second step of curing the matrix resin (C). In addition, the present invention is a preform containing the prepreg of the present invention as a lamination unit. In addition, the present invention is a fiber-reinforced resin molded article, which is formed by heating and pressurizing the preform of the present invention. In addition, the present invention is a fiber-reinforced resin molded body, which is formed by heating and expanding the preform of the present invention, and has voids. In addition, the present invention is a sandwich structure in which the fiber-reinforced resin molded body of the present invention is sandwiched between a pair of skin materials as the core material and integrated into one piece. Furthermore, this invention relates to an aircraft component, comprising the prepreg of this invention, the porous structure of this invention, the fiber-reinforced resin molded body of this invention, or the sandwich structure of this invention. [Effects of the Invention] The carbon fiber substrate of the present invention exhibits excellent conformability, softly following the mold shape during the initial stage of fiber-reinforced resin molding. Furthermore, it can prevent molding defects such as breakage during high-temperature, high-pressure processes involving impregnated matrix resins. Moreover, the carbon fiber substrate of the present invention can maintain the dispersed state of carbon fibers, easily yielding prepregs or fiber-reinforced resins with excellent heat resistance and porous structures. In addition, the prepreg of the present invention can achieve both heat resistance and isotropic mechanical properties. In this invention, the thickness direction of the prepreg is referred to as the out-of-plane direction, and the direction orthogonal to the out-of-plane direction is referred to as the in-plane direction. [Carbon fiber substrate] The carbon fiber substrate of the present invention comprises carbon fiber (A) and a resin composition (B) covering the surface of the carbon fiber (A). Among fibers known as reinforcing fibers, carbon fiber (A) also exhibits excellent mechanical properties and lightweight characteristics. Examples of carbon fiber (A) include polyacrylonitrile (PAN) based, rayon based, and pitch-based carbon fibers. From the viewpoint of mechanical properties and economy, PAN-based carbon fibers are preferred. Among PAN-based carbon fibers, from the viewpoint of the mechanical properties of the obtained fiber-reinforced matrix, carbon fibers with an elastic modulus of 200 GPa or higher are preferred. Furthermore, from the viewpoint of cost, recycled carbon fibers are also preferred. The thermal mass reduction rate of carbon fiber (A) at 400°C is preferably 1% by mass or less. More preferably, this thermal mass reduction rate is 0% by mass or more and 1% by mass or less. In this invention, carbon fiber (A), as described below, is preferably a discontinuous fiber, bonded together with a resin composition (B) to form a carbon fiber substrate. If the heat resistance of carbon fiber (A) is assumed to be low, decomposition gases may cause the interface between carbon fiber (A) and resin composition (B) to peel off, sometimes resulting in insufficient bonding. Therefore, it is preferable to set the thermal mass reduction rate of carbon fiber (A) within the aforementioned range. This thermal mass reduction rate can be calculated using the following formula: Thermal mass reduction rate (mass%) = [(M1-M2) / M1] × 100 Here, M1 is the sample mass (mg) after isothermal holding at 30°C for 10 minutes, and M2 is the sample mass (mg) after subsequent heating at 10°C / min to reach 400°C. The diameter of the carbon fiber (A) is preferably 1 μm to 20 μm, more preferably 3 μm to 10 μm, and even more preferably 6 μm to 8 μm. Within this range, the carbon fiber exhibits excellent dispersibility, effectively allowing for the coexistence of high porosity and mechanical properties as a carbon fiber substrate. Furthermore, the mechanical properties of the prepreg obtained from the carbon fiber substrate are also superior. In this invention, the carbon fibers (A) constituting the nonwoven fabric are preferably discontinuous fibers, more preferably with a number-average fiber length of 0.1 mm to 100 mm, further preferably 1 mm to 20 mm, and most preferably 3 mm to 10 mm. By setting it within this range, it is easy to increase the spacing between the fibers in the nonwoven fabric mesh structure, and it is easy to control the formation of pores. Regarding the number-average fiber length of the carbon fibers (A), 400 carbon fibers (A) can be randomly selected, and their fiber lengths can be measured using an optical microscope. The length can be calculated by dividing the total fiber length by the number of fibers measured. The carbon fiber (A) preferably contains at least 50% by mass of fibers with a length of 2 mm or more and 10 mm or less, more preferably 3 mm or more and 8 mm or less, and even more preferably 5 mm or more and 7 mm or less. This content is more preferably 80% by mass or more and 100% by mass or less, and even more preferably 100% by mass. By setting this range, the carbon fibers (A) constituting the nonwoven mesh are dispersed among themselves, making it easier to control the formation of pores or the isotropic fiber orientation. Sample collection methods for determining fiber length include, for example, the following: dissolution method using an organic solvent; melting method by heating and melting the resin components to prepare a thin-walled sample for observation; or the method of removing the resin components by burning them off in an electric furnace and then filtering and separating the remaining carbon fibers (A) before measuring them using an optical microscope. Carbon fibers (A) are formed into a nonwoven fabric. This has the following advantages: compared with woven fabrics or fiber substrates oriented in one direction, a high porosity can be designed, thereby increasing the impregnation content of the matrix resin (C) described later. In addition, by forming a nonwoven fabric from carbon fibers (A), even if byproduct gases are generated during the reaction of the resin composition (B) described later, the byproduct gases can be easily discharged, resulting in a rigid carbon fiber substrate. In Figure 1, carbon fibers (A) 1 are randomly dispersed as discontinuous fibers and form a nonwoven fabric with a mesh structure, thereby creating fine pores 3 between the fibers. Generally, compared to woven fabrics or fibrous substrates aligned in one direction, nonwoven carbon fiber substrates have lower strength and are more susceptible to breakage during melt impregnation with the matrix resin (C). Therefore, in this invention, carbon fibers with excellent mechanical properties are used. The nonwoven carbon fiber (A) can be manufactured using air-laid, carding, or papermaking methods. The mass ratio of carbon fiber (A) in the carbon fiber substrate is preferably 90% or more and 97% or less. This mass ratio of 90% or more ensures sufficient strength for the carbon fiber substrate, effectively preventing molding defects such as breakage during impregnation with the matrix resin (C). Furthermore, a mass ratio of 97% or less, more preferably 96% or less, and even more preferably 92% or less, prevents the carbon fiber (A) from converging into fiber bundles, thereby preventing insufficient dispersion of carbon fiber (A) in the prepreg obtained from the carbon fiber substrate. The resin composition (B) functions to maintain the nonwoven fabric shape or protect the carbon fibers (A) from friction by bonding the network structure of the carbon fibers (A) at their joints. In the right-hand figure of FIG1, the resin composition (B) 2 coats the surface of the carbon fibers (A), forming a carbon fiber (A) 4 coated with resin composition (B). The carbon fiber substrate of the present invention, bonded by the resin composition (B), can be prepreg-impregnated while maintaining the network structure such as fiber orientation or dispersion of the carbon fibers (A). From the viewpoint of maintaining the mesh structure of carbon fiber (A), the resin composition (B) preferably covers 50% to 100% of the surface area of ​​all carbon fibers (A) constituting the carbon fiber substrate, more preferably 80% to 100%, and even more preferably 100%. To achieve this state, it is preferable to form the carbon fiber (A) into a non-woven fabric-like aggregate and impregnate it with the resin composition (B). As described in the embodiments below, the coverage of resin composition (B) can be determined using the surface oxygen concentration (O / C) obtained by X-ray photoelectron spectrometry and the following formula: (Coverage of resin composition (B)) = {(O / C of carbon fiber (A)) - (O / C of carbon fiber substrate)} / {(O / C of carbon fiber (A)) - (O / C of resin composition (B))} × 100 (%). The porosity of the carbon fiber substrate of the present invention is preferably 50% to 99%. More preferably, the porosity is 80% to 99%, and even more preferably 90% to 99%. By setting the porosity within this range, a large impregnation content of the matrix resin (C) can be designed. Furthermore, the carbon fiber substrate of the present invention, despite its high porosity, also exhibits excellent mechanical properties at high temperatures, preventing molding defects such as breakage during the melt impregnation process of the matrix resin (C). The porosity of the carbon fiber substrate is the volume ratio of pores in the carbon fiber substrate, and can be calculated using the following formula: (Porosity of carbon fiber substrate) = {(Thickness of carbon fiber substrate [m]) - (Weight per unit area of ​​carbon fiber substrate [g / m²])} 2 ]) / (Density of the constituent components of the carbon fiber substrate [g / m 3])} / (thickness of carbon fiber substrate [m])×100[%). The tensile strength of the carbon fiber substrate of the present invention after heat treatment at 300°C for 5 minutes is more than 110% of that before heating. The tensile strength after heat treatment is preferably more than 200% and less than 600% relative to the tensile strength before heating, and more preferably more than 300% and less than 500%. As described later, the carbon fiber substrate of the present invention can be processed into a prepreg by melt impregnation with a matrix resin (C). In order to composite with the matrix resin (C), the heated and molten matrix resin (C) is impregnated into the pores of the carbon fiber substrate under high temperature and high pressure. However, during this impregnation process, defects such as breakage of the carbon fiber substrate sometimes occur. On the other hand, if a rigid carbon fiber substrate is used before the impregnation process, elastic rebound or breakage may sometimes occur when shaping the substrate according to the mold or the like. Therefore, by possessing the characteristics of being soft and conformable to molds at the start of the impregnation process, while simultaneously hardening without breakage even under pressure during impregnation, defects such as breakage can be prevented even during the high-temperature, high-pressure impregnation process of the matrix resin (C). The change in tensile strength before and after heat treatment is determined by measuring the tensile strength (Ta) of the carbon fiber substrate after heating at 300°C for 5 minutes at room temperature and the tensile strength (Tao) of the carbon fiber substrate in the same direction without the heat treatment at room temperature, and then calculating it using the following formula: Tensile strength after heat treatment relative to tensile strength before heating (%) = (Ta / Tao) × 100. Furthermore, the tensile strength [N / cm] of the carbon fiber substrate in this specification is set as follows: a test piece with a width of 50 mm and a length of 150 mm is cut from the carbon fiber substrate, 50 mm long portions at both ends are supported, and the remaining central 50 mm long portion is subjected to a tensile test at room temperature and a test speed of 3 m / min. The value is obtained by dividing the maximum point load obtained by the width of the test piece. The carbon fiber substrate of the present invention preferably exhibits excellent isotropy when impregnated with the matrix resin, meaning that the strength of the carbon fiber substrate in a certain direction is similar to that in the direction orthogonal to it. In cases of high anisotropy, the risk of concentrated breakage in the direction of weak tensile strength increases when impregnated with the matrix resin (C). As an indicator of this isotropy, a heat-treated tensile strength ratio (Ta / Tb) of 0.6 to 1.7 is preferred. This heat-treated tensile strength ratio (Ta / Tb) is obtained by dividing Ta by the tensile strength (Tb) measured at room temperature after heat treatment at 300°C for 5 minutes with respect to the orthogonal direction of the test. More preferably, this heat-treated tensile strength ratio (Ta / Tb) is 0.8 to 1.3, and even more preferably 0.9 to 1.1. Furthermore, the absolute values ​​of Ta and Tb are preferably 25 N / cm or more and 100 N / cm or less, and more preferably 25 N / cm or more and 45 N / cm or less. By setting this range, a strong and tough carbon fiber substrate is produced that does not break even when the matrix resin (C) is melt-impregnated. Furthermore, from the viewpoint of preventing breakage during the molding process of following the mold, the carbon fiber substrate of the present invention preferably exhibits excellent isotropy even before impregnation with the matrix resin. As this indicator, the tensile strength ratio (Tao / Tbo) before heat treatment is preferably 0.6 to 1.7, which is obtained by dividing the tensile strength before heat treatment, i.e., Tao, by the tensile strength (Tbo) measured at room temperature with respect to the orthogonal direction of the test direction. This tensile strength ratio (Tao / Tbo) before heat treatment is more preferably 0.8 to 1.3, and even more preferably 0.8 to 1.1. The absolute values ​​of Tao and Tbo are preferably 8 N / cm or more and 25 N / cm or less, more preferably 8 N / cm or more and 15 N / cm or less. By setting these values ​​within this range, a flexible carbon fiber substrate with high mold conformability is obtained. In this invention, resin composition (B) is preferably a resin composition that undergoes a dehydration condensation reaction with a heat loss rate of 5% to 50% by mass according to the following formula (1). Heat loss rate (mass%) = [(W1-W2) / W1]×100…(1) Here, W1 is the sample mass (mg) after heating from room temperature to 100°C at 10°C / min and then holding it isothermally for 30 minutes, and W2 is the sample mass (mg) after heating at 10°C / min to reach 250°C. Resin composition (B) has the function of hardening the carbon fiber substrate through a dehydration condensation reaction accompanied by heating. Furthermore, since the carbon fiber (A) is in a non-woven fabric form, byproduct gases generated during the dehydration condensation reaction of resin composition (B) can be easily expelled. By setting the thermal mass reduction rate of resin composition (B) within this range, the hardening of resin composition (B) and the expulsion of byproduct gases can be achieved evenly during the impregnation of the matrix resin (C) into the carbon fiber substrate. The mass and composition analysis of the gases generated during heating can be performed using a thermogravimetry-mass spectrometer (TG-MS). The thermal mass reduction rate accompanying the dehydration condensation reaction of resin composition (B) is preferably 5% to 30% by mass, and more preferably 5% to 20% by mass. When the resin composition (B) undergoes a dehydration condensation reaction, the generated byproduct gas contains water, preferably 5% or more, more preferably 50% or more, and even more preferably 100% water. By including a large amount of water in the byproduct gas, it is possible to prevent overheating caused by pressure during impregnation of the matrix resin (C) or to improve the release properties relative to the mold. From the viewpoint of the heat resistance of the obtained reaction products, the dehydration condensation reaction of the resin composition (B) as described above is more specifically preferred to be amide imidization, oxazolization, or imidazoleization. In the case of thermal decomposition reactions such as oxidation or free radical pyrolysis accompanied by mass reduction, the heat resistance or mechanical strength of the resin composition (B) decreases, sometimes resulting in a molding defect such as carbon fiber matrix breakage. From this perspective, resin composition (B) preferably includes at least one selected from polyamide, polyamide salt, polyamide ester, polyhydroxyamide, and polyaminoamide. Polyamide, polyamide salt, and polyamide ester are precursors to polyimide resins by undergoing amide imidization; polyhydroxyamide is a precursor to polybenzoxazole resins by undergoing oxazoleization; and polyaminoamide is a precursor to polybenzimidazole resins by undergoing imidization. Polyamides can be synthesized by reacting a tetracarboxylic dianhydride with an equal molar amount of a diamine compound. Specific synthetic methods include dissolving the diamine compound in a solvent such as N-methyl-2-pyrrolidone (NMP) and adding the tetracarboxylic dianhydride while stirring at room temperature. In this method, the carboxylic anhydride, which is the functional group of the tetracarboxylic dianhydride, and the amino group, which is the functional group of the diamine compound, undergo a ring-opening addition polymerization reaction at an equal molar ratio (1:1), thereby increasing the molecular weight. A carboxyl group derived from the carboxylic anhydride is generated as a byproduct. Polyamide salts are obtained by neutralizing this byproduct carboxyl group using a basic compound. Examples of basic compounds used for neutralization include ammonia, monoalkylamines, dialkylamines, trialkylamines, and tetraalkylammonium hydroxide. Polyamide esters can be synthesized by esterification of polyamide by reacting the byproduct carboxyl group with an alcohol. Examples of alcohols used herein include methanol, ethanol, 1-propanol, and 2-propanol. Esterification reduces the hydrolysis of polyamide, which is therefore preferable. Polyamide, polyamide salt, and polyamide ester are cyclized through intramolecular amide imidization caused by heating, transforming into polyamide while generating byproduct gases. Resin composition (B) may also be a mixture of polyamide, polyamide salt, and polyamide ester. From the viewpoint that it has high water solubility and can be used as an aqueous solution with relatively low equipment load, resin composition (B) is preferably polyamide. Examples of tetracarboxylic dianhydrides include: 3,3',4,4'-biphenyltetracarboxylic dianhydride <s-BPDA (3,3',4,4'-biphenyl tetracarboxylic dianhydride)>, 2,3,3',4'-biphenyltetracarboxylic dianhydride <a-BPDA (2,3,3',4'-biphenyl tetracarboxylic dianhydride)>, 2,2',3,3'-biphenyltetracarboxylic dianhydride <i-BPDA (2,2',3,3'-biphenyl tetracarboxylic dianhydride)>, pyromellitic dianhydride <PMDA (pyromellitic anhydride)>, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride <BTDA (3,3',4,4'-benzophenone tetracarboxylic dianhydride)>. dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 4,4'-oxydiphthalic anhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride <NTDA(1,4,5,8-naphthalenetetracarboxylic dianhydride)>, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride <6FDA(2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride)>, etc. Of these, 4,4'-(4,4'-isopropylidene diphenoxy)bis(phthalic anhydride) is preferred from the viewpoint that the obtained polyamide has good melt processability and that the carbon fiber substrate has good heat resistance and good adhesion to the matrix resin (C). Examples of diamine compounds include: p-phenylenediamine (PPD), m-phenylenediamine (MPD), and other phenylenediamines; diaminobenzoic acids such as 3,5-diaminobenzoic acid; diaminodiphenyl ethers such as 4,4'-diaminodiphenyl ether (ODA), 3,4'-diaminodiphenyl ether, and 3,3'-diaminodiphenyl ether; diaminodiphenylmethanes such as 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 3,3'-dichloro-4,4'-diaminodiphenylmethane, and 3,3'-dimethyl-4,4'-diaminodiphenylmethane; and diaminodibenzophenones such as 4,4'-diaminodibenzophenone. Of these, from the viewpoint of balancing the coatability and heat resistance of the obtained polyamide with its coatability on carbon fiber (A), phenylenediamine is preferred, and m-phenylenediamine is more preferred. Polyhydroxyamide is a precursor of polybenzoxazole resin. It is a prepolymer with amide bonds formed by the condensation polymerization of an aromatic diamine diol having an amino group and a hydroxyl group on an adjacent carbon atom in the aromatic ring with a dicarboxylic acid, an aromatic dicarboxylic acid ester, an aromatic dicarboxylic acid dihalogen compound, or a heterocyclic dicarboxylic acid derivative. Specific synthetic methods include mixing the aromatic diamine diol and the dicarboxylic acid derivative in a 1:1 molar ratio in a solvent such as NMP and stirring at 120°C for 12 hours. Polyhydroxyamide is further processed by heating at 200°C to 400°C, where the adjacent hydroxyl group condenses with the amide bond to form an oxazole ring, thereby forming a polybenzoxazole resin with both an oxazole ring and an aromatic ring within the molecule. Aromatic diamine diols include: 3,3'-dihydroxybenzidine, 3,3'-diamino-4,4'-dihydroxybiphenyl, 4,6-diaminoresorcinol, 2,5-diaminohydroquinone, 4,4'-diamino-3,3'-dihydroxydiphenyl ether, 3,3'-diamino-4,4'-dihydroxydiphenyl ether, 4,4'-diamino-3,3'-dihydroxydiphenylmethane, 3,3'-diamino-4,4'-dihydroxydiphenylmethane, 2,2-bis(3-amino-4-hydroxyphenyl)-propane, 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, and their hydrochlorides. Examples of dicarboxylic acid derivatives include isophthalic acid. Polyaminoamide is a precursor to polybenzimidazole resin. It is a prepolymer with amide bonds formed by the condensation polymerization of the diamine component of an aromatic tetraamine compound with dicarboxylic acids, aromatic dicarboxylic acid esters, aromatic dicarboxylic acid dihalides, heterocyclic dicarboxylic acids, and other dicarboxylic acid derivatives. Specific synthesis methods include mixing an aromatic tetraamine compound and a dicarboxylic acid derivative in a 1:1 molar ratio in a solvent such as NMP and stirring at 120°C for 12 hours. Polyaminoamide can also be heated at 200°C to 400°C, causing the adjacent amine groups to condense with the amide bonds to form an imidazole ring, thereby reacting to form a polybenzimidazole resin with both imidazole and aromatic rings within the molecule. Examples of aromatic tetraamine compounds include: 3,3'-diaminobenzidine, 1,2,4,5-tetra-aminobenzene, 2,3,5,6-tetra-aminopyridine, 3,3',4,4'-tetra-aminodiphenyl ether, 3,3',3,4'-tetra-aminodiphenylmethane, 3,3',4,4'-tetra-aminodiphenyldimethylmethane, and their hydrochlorides. Examples of dicarboxylic acid derivatives include isophthalic acid. Polybenzimidazole resins obtained by imidazoleization of polyaminoamides have glass transition temperatures exceeding 400°C, exhibiting superior heat resistance. The resin composition (B) preferably comprises at least one selected from polyamide, polyamide salt, polyamide ester, polyhydroxyamide, and polyaminoamide, and contains at least 50% by mass, more preferably at least 80% by mass, based on the total amount of these. More preferably, the resin composition (B) is composed solely of at least one selected from polyamide, polyamide salt, polyamide ester, polyhydroxyamide, and polyaminoamide. By setting it within this range, the resin composition (B) is closely bonded to the carbon fiber (A), resulting in a carbon fiber substrate with excellent heat resistance or mechanical strength. The resin composition (B) may suitably contain other additives, depending on the intended use and without prejudice to the purpose of the invention. Examples include: flame retardants, conductivity enhancers, antioxidants, antibacterial agents, insect repellents, mold release agents, antistatic agents, plasticizers, lubricants, colorants, pigments, dyes, foaming agents, etc. In the carbon fiber substrate of the present invention, the resin composition (B) is preferably 0.5 to 30 parts by weight relative to 100 parts by weight of carbon fiber (A). When the resin composition (B) is 0.5 parts by weight or more, more preferably 1 part by weight or more, further preferably 3 parts by weight or more, further preferably 5 parts by weight or more, and further preferably 8 parts by weight or more relative to 100 parts by weight of carbon fiber (A), the carbon fiber substrate exhibits excellent processability, making it less prone to breakage during handling. When the resin composition (B) is 30 parts by weight or less, more preferably 15 parts by weight or less, and further preferably 10 parts by weight or less relative to 100 parts by weight of carbon fiber (A), sufficient pores can be formed, facilitating the impregnation of the matrix resin (C). The resin composition (B) preferably comprises more than 50% of the surface area of ​​the coated carbon fiber (A). With this coverage of more than 50%, more preferably more than 80%, and even more preferably 100%, a strong and close interface between the carbon fiber (A) and the matrix resin (C) can be effectively formed in the prepreg of the present invention, which will be described later. In order to form this coating state, the manufacturing step of the carbon fiber substrate is preferably a step of impregnating a nonwoven fabric containing carbon fibers (A) with a resin composition (B). As described in the embodiments below, the coverage of resin composition (B) can be determined using the surface oxygen concentration (O / C) obtained by X-ray photoelectron spectroscopy and the following formula: (Coverage of resin composition (B)) (%) = {(O / C of carbon fiber (A)) - (O / C of carbon fiber substrate)} / {(O / C of carbon fiber (A)) - (O / C of resin composition (B))} × 100. By using the carbon fiber substrate of the present invention, molding defects such as breakage during impregnation with the matrix resin (C) can be prevented, and a prepreg with excellent expansion ratio can be obtained. In particular, the carbon fiber substrate of the present invention can be used to impregnate even high-heat resistant matrix resin (C) that requires high-temperature and high-pressure impregnation processes, while preventing molding defects such as breakage. As a result, a lightweight porous structure with excellent heat resistance can be easily obtained by using a high-heat resistant matrix resin (C). [Prepreg] The carbon fiber substrate of the present invention is suitable as a substrate for a prepreg. That is, the first state of the prepreg of the present invention is formed by impregnating a matrix resin (C) in a substrate derived from the carbon fiber substrate of the present invention. Furthermore, the second state of the prepreg of the present invention is formed by impregnating a matrix resin (C) in a substrate comprising carbon fiber (A) and an adhesive resin composition (D). The details of the carbon fiber (A) in the second state sample are the same as those of the carbon fiber (A) in the first state sample, i.e., the carbon fiber substrate of the present invention. The details of the matrix resin (C) in the first state sample and the matrix resin (C) in the second state sample are also the same. The adhesive resin composition (D) in the second state sample is preferably derived from the resin composition described as resin composition (B). The adhesive resin composition (B) in the first state sample and the adhesive resin composition (D) in the second state sample are collectively referred to as "adhesive resin composition". Hereinafter, unless otherwise specifically stated, the first state sample and the second state sample will be assumed to be the same. In Figure 2, a prepreg is formed in a substrate comprising carbon fiber (A) 1 and the adhesive resin composition 2, with fine pores formed inside. A matrix resin (C) 5 is impregnated in these pores. In the prepreg, the carbon fiber (A) 1 is randomly dispersed and exists in a nonwoven form forming a mesh structure, with the fine spaces between the fibers filled by the matrix resin (C) 5. Furthermore, the adhesive resin composition 2 is present at the interface between the carbon fiber (A) 1 and the matrix resin (C) 5. The mass ratio of carbon fiber (A) in the prepreg of the present invention is preferably 1% or more and 50% or less, more preferably 5% or more and 40% or less, and even more preferably 10% or more and 30% or less. If it is less than this range, the reinforcing effect of the carbon fiber substrate on the prepreg may sometimes be insufficient. If it is greater than this range, the carbon fiber substrate may sometimes suffer damage or other defects during impregnation with the matrix resin (C). Examples of resins included in the adhesive resin composition include phenolic resins, urea resins, melamine resins, polyimide resins, polybenzoxazole resins, polybenzimidazole resins, bismaleimide resins, benzoxazine resins, cyanate ester resins, etc., as well as copolymers, modified products, and resins blended from two or more of these. From the viewpoint of heat resistance, polyimide resins, polybenzoxazole resins, and polybenzimidazole resins are preferred. Furthermore, from the viewpoint of processability of the carbon fiber substrate, polyimide resins are more preferred, and among polyimide resins, polyether-polyimide resins having both ether groups and polyimide groups are particularly preferred. Furthermore, the adhesive resin composition is preferably a composition comprising a polymer or derivative thereof, wherein the polymer has at least one structure selected from the group consisting of an ether-nitroimide backbone, a benzoxazole backbone, a benzimidazole backbone, a benzoxazine backbone, and a cyanate ester backbone. By including this compound, the adhesive resin composition exhibits excellent heat resistance, and consequently, excellent adhesion to carbon fiber (A) or matrix resin (C). An ether-nitroimide backbone means having both ether groups and nitroimide groups; examples of polymers or derivatives having this backbone include polyether-nitroimide resins. A benzoxazole backbone is a structure in which an aromatic ring and an oxazole ring are bonded together on a common side; examples of polymers or derivatives having this backbone include polybenzoxazole resins. A benzimidazole backbone is a structure in which an aromatic ring and an imidazole ring are bonded together on a common side; examples of polymers or derivatives having this backbone include polybenzimidazole resins. A benzoxazine backbone is a polymer or derivative thereof containing a benzoxazine backbone; examples of benzoxazine resins. Examples of polymers or derivatives thereof having a cyanate ester backbone include cyanate ester resins. Furthermore, from the viewpoint of heat resistance and processability to carbon fiber substrates, the adhesive resin composition is preferably a polymer or derivative thereof having an ether amide backbone. The glass transition temperature of the adhesive resin composition in the first state sample is preferably 120°C or higher and 450°C or lower. Furthermore, the glass transition temperature of the adhesive resin composition (D) in the second state sample is 120°C or higher and 450°C or lower. More preferably, the glass transition temperature of the adhesive resin composition is 200°C or higher and less than 400°C, and even more preferably 200°C or higher and 350°C or lower. The adhesive resin composition has the function of maintaining the nonwoven fabric shape by bonding the mesh structure of the carbon fibers (A) at their joints during impregnation with the matrix resin (C), and exists at the interface between the carbon fibers (A) and the matrix resin (C) within the obtained prepreg. By having the glass transition temperature of the adhesive resin composition within the aforementioned range, it maintains the shape of the carbon fiber substrate as a solid in the initial stage of impregnation with the matrix resin (C), and then gradually softens, thereby dissolving with the molten matrix resin (C) to form a firmly bonded interface between the carbon fibers (A) and the matrix resin (C). The adhesive resin composition preferably has a thermal mass reduction rate of 5% by mass or less at 400°C. The adhesive resin composition functions to maintain the nonwoven fabric form by bonding the carbon fiber (A) at its joints through the network structure during impregnation with the matrix resin (C), and exists at the interface between the carbon fiber (A) and the matrix resin (C) within the obtained prepreg. Because the adhesive resin composition has a thermal mass reduction rate of 5% by mass or less, preferably 0% by mass or more and 3% by mass or less at 400°C, it exhibits high heat resistance and is not easily thermally decomposed, preventing decomposition gases from causing the interface between the carbon fiber (A) and the adhesive resin composition to peel off, thus reducing the bonding effect. This thermal mass reduction rate can be calculated using the following formula. Heat loss rate (mass%) = [(M1-M2) / M1]×100 Here, M1 is the sample mass (mg) after being held isothermally at 30℃ for 10 minutes, and M2 is the sample mass (mg) when the temperature is then increased at 10℃ / min to reach 400℃. The adhesive resin composition preferably comprises more than 50% of the surface area of ​​the coated carbon fiber (A). With this coverage of more than 50%, more preferably more than 80%, and even more preferably 100%, a strong and tight interface between the carbon fiber (A) and the matrix resin (C) can be effectively formed in the prepreg of the present invention, as described later. In order to form this coating state, the manufacturing step of the carbon fiber substrate is preferably a step of impregnating a nonwoven fabric containing carbon fibers (A) with a resin composition (B). As described in the embodiments below, the coverage of the adhesive resin composition can be determined using the surface oxygen concentration (O / C) obtained by X-ray photoelectron spectrometry and the following formula: (Coverage of adhesive resin composition) (%) = {(O / C of carbon fiber (A)) - (O / C of carbon fiber substrate)} / {(O / C of carbon fiber (A)) - (O / C of adhesive resin composition)} × 100. Generally, in prepregs, adhesive resins with high glass transition temperatures require high processing temperatures, making it difficult to coat the surface of reinforcing fibers with a high coverage rate. In this invention, as a means to solve this problem, the following method can be employed: coating the surface of carbon fiber (A) with the resin composition (B) as a precursor to the adhesive resin composition (D) and subjecting it to a heating reaction, thereby producing the adhesive resin composition (D). Alternatively, when using polyimide resin, polybenzoxazole resin, or polybenzimidazole resin as the adhesive resin composition (D), from the viewpoint of heat resistance and processability, the following method can be used more preferably: coating the resin composition (B) as its precursor onto carbon fiber (A), and then subjecting it to a dehydration condensation reaction, thereby producing the adhesive resin composition (D). The matrix resin (C) is preferably a thermoplastic resin. Examples of such thermoplastic resins include: polyester resins such as polyethylene terephthalate and polybutylene terephthalate; polyolefins such as polyethylene, polypropylene, polybutene, and modified polypropylene; polyamides such as polyoxymethylene, polyamide 6, and polyamide 66; polycarbonate; polymethyl methacrylate; polyvinyl chloride; polyphenylene sulfide; polyphenylene ether, modified polyphenylene ether, polyether ether, modified polyether ether, polyether ether, polyether ether ketone, polyether ketone, polyether ketone ketone, polyether ketone ketone, etc.; polyaryl ether ketones; polyaryl esters; polyether nitriles; and phenoxy resins. In addition, these thermoplastic resins may also be copolymers or modifiers, and / or resins made by blending two or more types. In terms of balancing processability and heat resistance, the matrix resin (C) preferably contains at least one component selected from the group consisting of polyetherketone, polyetheretherketone, polyetherketoneketone, and polycarbonate, and more preferably contains at least one component selected from the group consisting of polyetherketone, polyetheretherketone, and polyetherketoneketone, and more preferably contains polyetherketoneketone as a main component. Here, "main component" means accounting for 50% or more by mass of the constituent components, more preferably 80% or more and 100% or less by mass, and more preferably 100% by mass. The thermoplastic resin used as the matrix resin (C) in the first state sample preferably has a melting point of 250°C or higher and 400°C or lower. The thermoplastic resin used as the matrix resin (C) in the second state sample also has a melting point of 250°C or higher and 400°C or lower. By having the melting point of the thermoplastic resin be 250°C or higher, more preferably 280°C or higher, and even more preferably 300°C or higher, the heat resistance of the prepreg, or the material obtained by processing the prepreg, such as the porous structure described later, can be effectively improved. Furthermore, by having the melting point of the thermoplastic resin be 400°C or lower, more preferably less than 400°C, even more preferably 360°C or lower, and even more preferably 350°C or lower, the processability of the prepreg can be effectively improved. The glass transition temperature of the thermoplastic resin used as the matrix resin (C) is preferably 80°C or higher and 250°C or lower, more preferably 100°C or higher and 230°C or lower, and even more preferably 140°C or higher and 180°C or lower. By using a glass transition temperature of 80°C or higher, preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 140°C or higher, the heat resistance of the prepreg or the porous structure described later can be effectively improved. Furthermore, by using a glass transition temperature of 250°C or lower, preferably 230°C or lower, more preferably 200°C or lower, and even more preferably 180°C or lower, the compatibility with the adhesive resin composition is excellent, and the tensile strength of the obtained prepreg can be effectively improved. The matrix resin (C) is also preferably composed of other fillers or additives, depending on the intended use and without prejudice to the purpose of the invention. Examples include: inorganic fillers, flame retardants, conductivity-improving agents, crystallizing nucleating agents, ultraviolet absorbers, antioxidants, vibration damping agents, antibacterial agents, insect repellents, deodorizing agents, anti-coloring agents, heat stabilizers, mold release agents, antistatic agents, plasticizers, lubricants, colorants, pigments, dyes, foaming agents, foaming agents, coupling agents, etc. The amount of matrix resin (C) in the prepreg is preferably 5 to 1000 parts by weight relative to 100 parts by weight of carbon fiber (A), more preferably 50 to 700 parts by weight, and even more preferably 100 to 500 parts by weight. If it is less than this range, the mechanical properties of the prepreg or the porous structure formed therefrom may be insufficient. If it is greater than this range, the amount of matrix resin (C) is too large relative to the pore volume, and during the melt impregnation of the matrix resin (C), it may sometimes cause defects such as breakage of the carbon fiber substrate. The prepreg of the first state sample preferably has a minimum tensile strength of 100 MPa or more. The prepreg of the second state sample has a minimum tensile strength of 100 MPa or more. The minimum tensile strength of the prepreg is preferably 200 MPa or more, more preferably 240 MPa or more, and even more preferably 260 MPa or more and 400 MPa or less. Generally, the tensile strength of a prepreg is strongly influenced by the fiber orientation of the encapsulating carbon fiber matrix. Even within the same plane of the prepreg, the tensile strength is higher in directions where the fiber orientation is more biased towards and nearly parallel to the matrix, and lower in directions orthogonal to it. Therefore, there are cases where the tensile strength of the prepreg is high in one direction but low in directions orthogonal to it. From this perspective, to obtain a prepreg with high isotropic mechanical properties in the in-plane direction, it is ideal to design the tensile strength based on the minimum value. The minimum tensile strength can be determined using test pieces cut from the prepreg in four directions: 45°, 90°, and -45°, with any one of the in-plane directions of the prepreg as the reference 0°. For each direction, the number of measurements is set to n=5, and the minimum value among all measurements (n=20) is taken as the minimum tensile strength. The prepreg of the present invention preferably has a maximum tensile strength of 260 MPa or more, and more preferably 260 MPa or more and 400 MPa or less. Generally, the more carbon fibers in the inner carbon fiber matrix that are oriented closer to the stretching direction, the higher the tensile strength of the prepreg. By setting the maximum tensile strength within this range and designing it to the same degree as the minimum tensile strength, a prepreg with more isotropic mechanical properties can be formed. The maximum tensile strength can be determined using test pieces cut from the prepreg in four directions: 45°, 90°, and -45°, with any one of the in-plane directions of the prepreg as the reference 0°. For each direction, the number of measurements is set to n=5, and the largest value among all measurements (n=20) is taken as the maximum tensile strength. The prepreg of the present invention preferably has a ratio of the maximum tensile strength to the minimum tensile strength of 1 to 3, more preferably 1 to 1.5, further preferably 1 to 1.2, and most preferably 1 to 1.1. By setting it within this range, the mechanical properties are identical in any direction in the in-plane direction of the prepreg, and a prepreg with isotropically excellent mechanical properties can be produced. This value can be obtained by dividing the maximum tensile strength of the prepreg by the minimum tensile strength of the prepreg. Furthermore, the prepreg of the present invention preferably has an average tensile strength of 100 MPa or more, more preferably 200 MPa or more, even more preferably 240 MPa or more, and particularly preferably 260 MPa or more and 400 MPa or less. By setting it within this range, a prepreg with isotropically high mechanical properties can be obtained. This average tensile strength can be measured using test pieces obtained by cutting out a total of four directions—45°, 90°, and -45°—with one of the in-plane directions of the prepreg as a reference (0°). For each direction, the number of measurements is set to n=5, and the average value of all measured values ​​(n=20) is taken as the average tensile strength. Furthermore, from the viewpoint of obtaining prepregs with isotropic mechanical properties, it is preferable to set the deviation of in-plane tensile strength to 20% or less, more preferably 10% or less, and even more preferably 5% or less. The deviation of tensile strength is the coefficient of variation (CV), which can be calculated from the average and standard deviation of all measured values ​​(n=20) using the following formula. More specifically, it can be measured using the method described in the embodiments below. Deviation of tensile strength (%) = [(Standard deviation of tensile strength) / (Average tensile strength)] × 100. The prepreg of the present invention preferably has a thickness of 0.1 mm or more and 5 mm or less at 400°C, more preferably 0.1 mm or more and 2 mm or less, and even more preferably 1 mm or more and 2 mm or less. This thickness at 400°C corresponds to the thickness of the prepreg after springback expansion due to the encapsulated carbon fiber substrate. The prepreg can be used as a single layer or in multiple layers for molding applications corresponding to desired thicknesses. By controlling the thickness of the prepreg during expansion within this range, it is possible to mold porous bodies of various thicknesses, which is therefore preferable. Regarding the thickness at 400°C, the thickness of the prepreg heated on a 400°C heating plate for 5 minutes can be measured non-contactly using a laser displacement meter, and the average value can be used to calculate the thickness. The prepreg of the first state preferably has a thickness deviation of 10% or less at 400°C. The prepreg of the second state preferably has a thickness deviation of 10% or less at 400°C. The thickness deviation at 400°C is preferably 8% or less, more preferably 0% or more and 5% or less. When the prepreg of the present invention is heated to 400°C, the matrix resin (C) melts and rebounds, expanding within the voids. This expansion originates from the carbon fiber substrate encapsulated within the prepreg, and the deviation in thickness after expansion becomes an indicator of isotropy in the out-of-plane direction. That is, if the difference in thickness after expansion at a certain location is large relative to the surrounding area, it can be said that the fiber orientation or fiber concentration of the encapsulated carbon fiber substrate is different. In the case of a prepreg that is isotropic in the out-of-plane direction, the thickness after expansion is generally equal, resulting in a smaller thickness deviation. Furthermore, the fiber-reinforced resin molded body or void body described later is an integrally molded article formed by bonding the prepreg that constitutes the preform between layers. By producing a prepreg that has expansibility and small thickness deviation during expansion, gaps can be avoided in the interlayer, resulting in a well-bonded fiber-reinforced resin molded body or void body. The thickness deviation at 400°C is represented by the following CV value (coefficient of variation): the thickness of the prepreg heated on a 400°C heating plate for 5 minutes is measured non-contactly using a laser displacement meter. The CV value (coefficient of variation) can be calculated from the average and standard deviation of the thickness using the following formula, or more specifically, by the method described in the examples below. Thickness deviation at 400°C (%) = [(Standard deviation of thickness at 400°C) / (Average thickness at 400°C)] × 100. The prepreg of the present invention is heated to 400°C, where a matrix resin (C) with a melting point of 250°C or higher and 400°C or lower, preferably 280°C or higher and 400°C or lower, melts, resulting in an expanded state in which the resilience of the carbon fiber substrate is maximized. By setting this state, the fiber orientation or fiber density of the carbon fiber substrate encapsulated within the prepreg can be determined. That is, the thickness of the prepreg after expansion at 400°C is thinner in areas where the fiber orientation of the carbon fibers (A) is biased and converges into fiber bundles, or in areas where the carbon fiber substrate is damaged and the concentration of carbon fibers (A) becomes thinner. The distribution of these areas can be detected as a deviation in the thickness after expansion. Such defects in the carbon fiber substrate are easily generated during the impregnation of the matrix resin (C), and as a method to confirm these defects after prepreg treatment, the measurement of the thickness deviation at 400°C is effective. Furthermore, defects in the carbon fiber substrate encapsulated within the prepreg can affect the tensile strength of the prepreg. Therefore, for example, by ensuring that the thickness deviation at 400°C is less than 10%, a prepreg with high average tensile strength can be produced, or a prepreg that simultaneously achieves isotropy, i.e., a small ratio of the maximum to the minimum tensile strength and a high maximum tensile strength can be produced. From the viewpoint of short impregnation distance and excellent productivity, the prepreg of the present invention is preferably manufactured by impregnating a matrix resin (C) in the out-of-plane direction. In this manufacturing method, firstly, the carbon fiber substrate resists the compression that occurs accompanying the impregnation of the matrix resin (C). Under this repulsive force, the matrix resin (C) flows in the in-plane direction. During this resin flow, a force that stretches the carbon fiber substrate in the in-plane direction is exerted. When using the previous carbon fiber substrate, breakage occurs relative to the force in the in-plane direction, resulting in poor forming. By using the carbon fiber substrate of the present invention, such breakage can be prevented. As a method for manufacturing the prepreg, a method comprising the following steps can be listed: a heating impregnation step, in which the matrix resin (C) is plasticized by melting and impregnating it into the pores of the carbon fiber substrate; and a cooling curing step, in which the impregnated matrix resin (C) is cured. In this manufacturing method, as a method for composited with the carbon fiber substrate, methods include: directly injecting molten matrix resin (C) into the carbon fiber substrate; or depositing film-like, powder-like, or fibrous matrix resin (C) onto the carbon fiber substrate, and then impregnating the matrix resin (C) by heating and melting. From the viewpoint of suppressing uneven concentration or fiber orientation bias in the prepreg and facilitating manufacturing, a method of heating and pressurizing the laminate of film-like matrix resin (C) and carbon fiber substrate is preferred. The temperature during the heating and impregnation step is preferably 250°C to 450°C, more preferably 280°C to 400°C, and even more preferably 300°C to 400°C. The pressure during the heating and impregnation step is preferably 0.5 MPa to 20 MPa, more preferably 5 MPa to 15 MPa. Even under these high temperature and high pressure conditions, prepregs can be manufactured efficiently using the carbon fiber substrate of the present invention. The heating and impregnation time is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting the heating time within this range, the hardening of the carbon fiber substrate can be carried out in parallel with the impregnation of the matrix resin (C), thus achieving a strong and tight bond. The temperature for the cooling and curing step is preferably 20°C to 250°C, more preferably 100°C to 200°C. The pressure for the cooling and curing step is preferably 0.5 MPa to 20 MPa, more preferably 5 MPa to 15 MPa. The time for the cooling and curing step is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting these ranges, the prepreg can be manufactured efficiently. As equipment for the manufacturing method of this prepreg, a compression molding machine or a twin-belt press is preferable. In batch processing, the former can be used, and productivity can be improved by using an intermittent pressing system consisting of two or more machines arranged in parallel for heating and cooling. In continuous processing, the latter allows for easy continuous processing, thus providing excellent continuous productivity. The prepreg, manufactured by the method illustrated above, contains a carbon fiber substrate that has accumulated out-of-plane repulsive forces due to out-of-plane compression during impregnation with the matrix resin (C). Therefore, when the matrix resin (C) melts, the carbon fiber substrate releases these repulsive forces, exhibiting a rebound phenomenon that attempts to restore its original porosity. In other words, the prepreg possesses out-of-plane expansibility, thereby enabling it to be formed into a porous structure. The prepreg of the present invention preferably expands at an average out-of-plane expansion rate of 150% to 1,000% when the matrix resin (C) melts, more preferably 200% to 800%, and even more preferably 450% to 700%. This value is the expansion rate that can be calculated by (thickness of the expanded porous structure [mm]) / (thickness of the prepreg before expansion [mm]) × 100 [%), and is the arithmetic mean of 5 samples. The prepreg of the present invention contains a carbon fiber substrate, which is in a state where it has accumulated out-of-plane repulsive force due to out-of-plane compression during impregnation with the matrix resin (C). Therefore, when the matrix resin (C) melts, the carbon fiber substrate releases the repulsive force, resulting in a rebound phenomenon that aims to return to its original thickness. That is, this prepreg has out-of-plane expansibility, thereby enabling it to form a porous body by drawing in air during expansion. [Porous Structure] The prepreg of the present invention can be effectively formed into a porous structure by utilizing its expansion during heating. That is, the porous structure of the present invention is formed from the prepreg of the present invention. In Figure 3, the carbon fiber substrate, comprising carbon fibers (A) and a resin composition (B) covering its surface, expands outward due to its resilience. The resulting pores, as micropores, are generated within the matrix resin (C) as a result of this volume expansion. This structure, serving as a porous structure, achieves lightweighting while also leveraging the reinforcing effect of the carbon fiber substrate. Thus, by using a structure where the pores within the porous structure are reinforced by the carbon fiber substrate, both the increased porosity leading to lightweighting and the suppression of pore deformation can be achieved simultaneously, resulting in a high level of both lightweighting and mechanical properties. A preferred method for manufacturing the porous structure includes: a first step of melting and expanding the matrix resin (C); and a second step of curing the matrix resin (C). The first step is to plasticize the matrix resin (C) by melting and selectively expand it in the outward direction by utilizing the resilience of the carbon fiber substrate. In the first step, to melt and expand the matrix resin (C), the temperature at which the prepreg is heated is preferably 250°C to 450°C, more preferably 300°C to 400°C. The pressure in the first step is preferably 0 MPa to 5 MPa, more preferably 0.1 MPa to 3 MPa. The time taken in the first step is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting the time within this range, the prepreg expands efficiently, resulting in a porous structure with excellent mechanical properties. The second step is to fix the resilient carbon fiber substrate by curing the matrix resin (C) to form a porous structure. The temperature in the second step is preferably 20℃ to 250℃, more preferably 100℃ to 200℃. The pressure in the second step is preferably 0 MPa to 5 MPa, more preferably 0.1 MPa to 3 MPa. The time in the second step is preferably 0.5 minutes to 60 minutes, more preferably 1 minute to 30 minutes. By setting these ranges, the expansion ratio can be controlled, resulting in a porous structure with excellent mechanical properties. Furthermore, in either the first or second step, it is preferable to adjust the thickness of the molded article. From the viewpoint of ease of manufacturing, methods for thickness control that constrain the thickness using a metal plate or similar material, or methods that directly control the thickness by adjusting the applied pressure, are preferred. As equipment for implementing this method, a pressing machine or a double-belt pressing machine is preferable. In batch processing, the former can be used, and productivity can be improved by using an intermittent pressing system consisting of two or more machines arranged in parallel for heating and cooling. In continuous processing, the latter allows for easy continuous processing, thus providing excellent continuous productivity. In this invention, the porous structure has micropores with an average pore diameter of 500 μm or less. The average pore diameter is preferably 200 μm or less, more preferably 10 μm or more and 150 μm or less, and even more preferably 30 μm or more and 100 μm or less. If it is less than this range, the lightweight effect of the porous structure due to expansion is sometimes insufficient; if it is greater than this range, the mechanical properties of the porous structure sometimes decrease. The average pore diameter here, as shown in Figure 4 (a schematic diagram of the surface of the porous structure), is the arithmetic mean of the lengths obtained by measuring the diameter 8 of the largest circle tangent to the pores within the observation image at 50 randomly selected locations. Surface observation of the porous structure can be performed, for example, by a scanning electron microscope (SEM). The density of the porous structure of the present invention is preferably 0.01 g / cm³. 3 Above and 0.8 g / cm 3 The preferred value is 0.1 g / cm³. 3 Above and 0.5 g / cm 3 The optimal value is 0.2 g / cm³. 3 Above and 0.3 g / cm 3 The following applies. If the density is less than this range, the reinforcing effect of the carbon fiber substrate as a porous structure is sometimes insufficient; if it is greater than this range, the lightweighting effect as a porous structure is sometimes insufficient. The density here [g / cm³] 3 [] is the sample mass [g] of the porous structure divided by the volume [cm³] calculated based on the sample's perimeter. 3 The value obtained can be used as the arithmetic mean of the results measured in 5 randomly selected samples. The porous structure of the present invention preferably has a flexural strength at 140°C that is at least 80% of the flexural strength at 30°C, more preferably at least 90% and less than 100%. This value can be calculated as the "retention rate of flexural strength at 140°C" using the following formula: (Retention rate of flexural strength at 140°C) = (Flexural strength at 140°C [MPa]) / (Flexural strength at 30°C [MPa]) × 100 [%]. The flexural strength is determined by a three-point bending test according to Japanese Industrial Standards (JIS) K 7171 (2016), and is set as the arithmetic mean of five samples at each level. A constant temperature bath is used for temperature control at each level, and the tests are conducted at 30°C and 140°C respectively. If the retention rate of flexural strength when heated to 140°C is within the aforementioned range, a porous structure with excellent heat resistance that can be used as a structural material even in high-temperature environments such as 100°C is preferred. The prepreg of the present invention can be used as a single layer or as a preform comprising a lamination unit. Because the prepreg of the present invention has isotropic mechanical properties, it eliminates the need for lamination to compensate for weaker directions, as is the case with quasi-isotropic lamination, thus offering the advantage of high design freedom in lamination structures. Therefore, it reduces labor and time in the lamination process, thereby alleviating economic burden. [Preform] The preform of the present invention comprises the prepreg of the present invention as a lamination unit. The number of laminated sheets in the preform is preferably 2 or more and 50 or less, more preferably 2 or more and 10 or less. The preform can be formed as a "fiber-reinforced resin molded body with voids" (hereinafter, sometimes referred to as a "void body") or a "fiber-reinforced resin molded body without voids". [Fiber-reinforced resin molded body] The void body, which is one of the forms of the fiber-reinforced resin molded body of the present invention, is formed by heating and expanding the preform of the present invention, that is, by fusing the multilayer prepreg constituting the preform of the present invention in an expanded state. Methods for manufacturing the porous body include steps (1-A) of melting the matrix resin (C) and (2-A) of curing the matrix resin (C) while controlling the thickness of the molded article. The prepreg or preform of the present invention, as a laminate, has out-of-plane expansion, and therefore, by controlling the thickness of the molded article while molding, it is possible to form a fiber-reinforced resin molded article with internal pores. Step (1-A) is to plasticize the matrix resin (C) by melting and selectively expand it in the out-of-plane direction by utilizing the resilience of the carbon fiber substrate. The temperature in step (1-A) is preferably 250°C to 450°C, more preferably 280°C to 450°C, and even more preferably 300°C to 400°C. The pressure in step (1-A) is preferably 0 MPa to 5 MPa, more preferably 0.1 MPa to 3 MPa. The time in step (1-A) is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By using the conditions in step (1-A) within the aforementioned range, the prepreg constituting the preform can expand efficiently, thereby firmly bonding the layers of the prepreg together. Step (2-A) is a step of immobilizing the resilient carbon fiber substrate by curing the matrix resin (C) to form a void body. The temperature in step (2-A) is preferably 20°C to 280°C, more preferably 20°C to 250°C, even more preferably 20°C to 200°C, and even more preferably 20°C to 100°C. The pressure in step (2-A) is preferably 0 MPa to 3 MPa, more preferably 0 MPa to 1 MPa. The time in step (2-A) is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting the conditions in step (2-A) within the aforementioned ranges, the expansion rate can be controlled, resulting in a porous body with excellent mechanical properties and lightweight characteristics. In addition, in step (2-A), the thickness of the molded article is controlled. From the viewpoint of ease of manufacturing, the preferred methods for thickness control are methods such as using a metal plate to constrain the thickness, or methods that directly control the thickness by adjusting the pressure. As equipment for realizing the manufacturing method of the void body, a pressing machine can be preferably used. By using an intermittent pressing system consisting of two or more machines arranged in parallel for heating in step (1-A) and cooling in step (2-A), productivity can be improved. The void body of the present invention is preferably manufactured with an average out-of-plane expansion rate of 150% to 1,000% relative to the preform used in molding, more preferably 200% to 800%, and even more preferably 300% to 400%. This value is the expansion rate that can be calculated by (thickness of void body [mm]) / (thickness of preform [mm]) × 100 [%), and is the arithmetic mean of 5 samples. By setting it within this range, a void body with an excellent balance between lightweight and mechanical properties can be obtained. The density of the void body in this invention is preferably 0.01 g / cm³. 3 Above and 0.8 g / cm 3 The preferred value is 0.1 g / cm³. 3 Above and 0.5 g / cm 3 The optimal value is 0.3 g / cm³. 3 Above and 0.5 g / cm 3 The following. By setting this range, a porous body with an excellent balance between lightweight and mechanical properties can be obtained. The density here [g / cm³] 3 [] is the sample mass [g] of the void body divided by the volume [cm³] calculated based on the sample's perimeter. 3 The value obtained is the arithmetic mean of the results measured in 5 randomly selected samples. One of the forms of fiber-reinforced resin molded articles of the present invention, a void-free fiber-reinforced resin molded article, is formed by heating and pressurizing the preform of the present invention, that is, by fusing the multilayered prepreg constituting the preform of the present invention in a non-expanded state. As a method for manufacturing the aforementioned void-free fiber-reinforced resin molded article, examples include a step (1-B) of melting the matrix resin (C) and a step (2-B) of curing the matrix resin (C) while pressing the rebound force of the prepreg. The prepreg or preform as a laminate of the present invention has out-of-plane expansion, but by molding under high pressure under the rebound force, it is possible to form a non-expanded fiber-reinforced resin molded article. Step (1-B) is the step of plasticizing the matrix resin (C) by melting. The temperature in step (1-B) is preferably 250°C to 450°C, more preferably 280°C to 450°C, and even more preferably 300°C to 400°C. The pressure in step (1-B) is preferably 0 MPa to 5 MPa, more preferably 0.1 MPa to 3 MPa. The time in step (1-B) is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting the conditions in step (1-B) within the aforementioned ranges, the layers of the prepreg constituting the preform can be firmly bonded together. Step (2-B) is a step of forming a fiber-reinforced resin molded body by pressing the prepreg with its elasticity while curing the matrix resin (C), thereby creating a laminated prepreg in an integral state without expansion. The temperature in step (2-B) is preferably 20°C to 280°C, more preferably 20°C to 250°C, even more preferably 20°C to 200°C, and even more preferably 20°C to 100°C. The pressure in step (2-B) is preferably 3 MPa to 10 MPa, more preferably 5 MPa to 10 MPa. The time in step (2-B) is preferably 0.5 minutes to 60 minutes, more preferably 5 minutes to 30 minutes. By setting the conditions in step (2-B) within the aforementioned ranges, the expansion rate can be controlled, and fiber-reinforced resin molded articles with excellent mechanical properties can be obtained. As equipment for the manufacturing method of the fiber-reinforced resin molded article, a compression molding machine can be preferably used. By using an intermittent compression system consisting of two or more machines arranged in parallel for heating in step (1-B) and cooling in step (2-B), productivity can be improved. The fiber-reinforced resin molded article, free of voids, preferably has an average out-of-plane expansion rate of 90% to 150% relative to the preform used in molding, more preferably 100% to 120%, and even more preferably 100% to 105%. This value is the expansion rate that can be calculated by (thickness of the fiber-reinforced resin molded article [mm]) / (thickness of the preformed article [mm]) × 100 [%), and is the arithmetic mean of 5 samples. By setting it within this range, a fiber-reinforced resin molded article with excellent mechanical properties can be obtained. The density of the fiber-reinforced resin molded article without voids is preferably 0.8 g / cm³. 3 Above and 2.0 g / cm 3 The preferred value is 1.2 g / cm³. 3 Above and 1.5 g / cm 3 The following. By setting this range, fiber-reinforced resin molded articles with excellent mechanical properties can be obtained. The density here [g / cm³] 3 [ ] is the sample mass [g] of the fiber-reinforced resin molded body divided by the volume [cm³] calculated based on the sample's outer perimeter. 3 The value obtained is the arithmetic mean of the results measured in 5 randomly selected samples. The fiber-reinforced resin molded articles of the present invention have excellent processability, and are therefore preferably molded into corrugated or honeycomb shapes. Here, a corrugated shape refers to a shape consisting of multiple curved or continuously curved corrugated plates. A honeycomb shape refers to a structure formed by using a corrugated fiber-reinforced resin molded article as chamber walls and joining multiple chamber walls at curved or bent protrusions; it is an assembly of multiple hollow columnar chambers divided by chamber walls. The shape of the bottom surface of the hollow columnar chambers is preferably triangular, quadrangular, or hexagonal, and from the viewpoint of mechanical properties, a hexagonal shape is more preferred. [Sandwich Structure] The sandwich structure of the present invention is formed by sandwiching the fiber-reinforced resin molded body of the present invention as a core material between a pair of skin materials and integrating them together. The fiber-reinforced resin molded body of the present invention has excellent heat resistance, therefore, when used as a core material, deformation can be suppressed even in the heating and pressurizing steps used to join the skin materials, resulting in a sandwich structure with excellent dimensional accuracy. In addition, the sandwich structure of the present invention has an excellent balance between lightweight and mechanical properties. There are no particular limitations on the skin material, and examples include: prepreg or fiber-reinforced resin molded body, metal sheet, resin sheet, etc. [Aircraft Components] The prepreg of the present invention, the porous structure or fiber-reinforced resin molded body using the prepreg, and the sandwich structure are preferably used in aircraft components, satellites, urban air mobility (UAM), and drones, as well as automotive components, railway vehicle components, ship components, civil engineering components, electronic machine frames, sporting goods, etc. In particular, the present invention is preferably used in aircraft components requiring both heat resistance and reliability. [Examples] The materials used in the embodiments and comparative examples are as described below. [PEKK] uses a crystalline polyetherketone ketone containing "Kepstan" (registered trademark) 6003 manufactured by Arkema (stock), and with a melting point of 305°C and a glass transition temperature of 157°C as determined according to JIS K7121 (2012). [PEEK] uses a crystalline polyetheretherketone (PEEK381G, manufactured by Victrex, a registered trademark) with a melting point of 334°C and a glass transition temperature of 143°C as determined by JIS K7121 (2012). [Carbon fiber nonwoven fabric] (CF-1) is a carbon fiber bundle with a total of 12,000 monofilaments obtained by spinning, calcining, and surface oxidation of a copolymer mainly composed of polyacrylonitrile. The characteristics of this carbon fiber bundle are: a tensile modulus of elasticity of 220 GPa as determined by JIS R7608 (2007), and a circular cross-section with a single fiber diameter of 7 μm. Using the aforementioned carbon fiber bundles, 6 mm long short carbon fibers are obtained by cutting them with a cartridge cutter. A dispersion containing water and a surfactant (Nacalai Tesque (semi-well reagent), polyoxyethylene lauryl ether (trade name)) at a concentration of 0.1% by mass is prepared. This dispersion is used with the short carbon fibers to produce carbon fiber nonwoven fabric. The manufacturing apparatus includes a 1000 mm diameter cylindrical container with an opening and closing cock at the bottom, serving as a dispersion tank, and a linear conveyor (inclined at 30°) connecting the dispersion tank and the papermaking tank. A stirrer is attached to an opening on the upper surface of the dispersion tank, through which the short carbon fibers and the dispersion medium (dispersion medium) can be added. The papermaking tank includes a mesh conveyor with a 500 mm wide papermaking surface at the bottom, and a conveyor connected to the mesh conveyor for transporting the carbon fiber nonwoven fabric (papermaking substrate). Papermaking is performed with the carbon fiber concentration in the dispersion set at 0.05% by mass. In addition, the papermaking conditions were adjusted to disperse the carbon fibers as monofilaments, and to ensure isotropic dispersion in the in-plane direction. The paper-made substrate was then dried in a drying oven at 200°C for 30 minutes to produce a thickness of 1.68 mm and a unit area weight of 100 g / m². 2 The carbon fiber nonwoven fabric CF-1 has a thermal mass reduction rate of 0%. (CF-2) The carbon fiber nonwoven fabric CF-2 is obtained in the same manner as CF-1, except that the length cut using a box cutter is changed to 3 mm. The area weight of CF-2 is 100 g / m². 2 The thickness is 0.85 mm, and the thermal mass reduction rate is 0 mass. [Polyamide (PAMC)] A polyamide is prepared as follows: 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), a tetracarboxylic dianhydride, is reacted with m-phenylenediamine, a diamine compound, in a 1:1 molar ratio to obtain polyamide. This polyamide is further neutralized with N,N-dimethyl-2-aminoethanol, a trialkylamine, to form the polyamide. The compound is diluted with purified water to prepare a 10% by mass aqueous solution for use. [Polyvinyl alcohol (PVA)] Polyvinyl alcohol (manufactured by Fujifilm and Hikari Pure Chemicals, Ltd., polyvinyl alcohol 500, fully saponified type) was used. The compound was diluted with purified water to prepare a 10% by mass aqueous solution for use. Polyamine amide (PAA) was synthesized by reacting 3,3'-diaminobenzidine, an aromatic tetraamine compound, with isophthalic acid, a dicarboxylic acid derivative, in a 1:1 molar ratio. The polyamine reaction was carried out in NMP solvent. A 10% by mass NMP solution was prepared for use. The evaluation methods for the structure, physical properties, etc. in each embodiment / comparative example are described below. [Porosity of Carbon Fiber Substrate] The porosity of a carbon fiber substrate is the volume ratio of pores in the carbon fiber substrate, and it is calculated using the thickness of the carbon fiber substrate, its unit area weight, and the density of its constituent components, by the following formula. The density can be calculated using JISK0061 (2001). (Porosity of carbon fiber substrate) = {(Thickness of carbon fiber substrate [m]) - (Unit area weight of carbon fiber substrate [g / m³])} 2 ]) / (Density of the constituent components of the carbon fiber substrate [g / m 3 ])} / (thickness of carbon fiber substrate [m])×100[%). [Coating percentage of resin composition (B)] The coating percentage of resin composition (B) on the carbon fiber substrate can be calculated using the surface oxygen concentration (O / C) obtained by X-ray photoelectron spectrometry and the following formula: Coating percentage of resin composition (B) (%) = {(O / C of carbon fiber (A)) - (O / C of carbon fiber substrate)} / {(O / C of carbon fiber (A)) - (O / C of resin composition (B))} × 100. In the samples used here, in addition to the carbon fiber substrate, samples were also used, namely, carbon fiber (A) monomer before coating with resin composition (B) and samples obtained by forming a 0.1 mm thick film of resin composition (B) from monomer. The surface oxygen concentration (O / C) of each sample was determined according to the following procedure. First, the sample was cut into pieces 20 mm wide and 20 mm long, and placed on a sample support stage, using AlKα... 1,2 As an X-ray source, the sample chamber is maintained at 1×10 -8 Torr was measured at a photoelectron emission angle of 45°. As a correction value for the peak generated by the charge during the measurement, C... 1s The binding energy of the main peak (peak) is consistent with 284.6 eV. The carbon number is calculated based on the baseline of a straight line drawn in the range of 282 eV to 296 eV. 1s The number of oxygen atoms is determined by the peak area. The number of oxygen atoms is calculated based on the baseline of a straight line drawn in the range of 528 eV to 540 eV. 1s The peak area is used to determine the oxygen concentration. The surface oxygen concentration (O / C) is determined by dividing the number of oxygen atoms by the number of carbon atoms, i.e., the atomic ratio. The X-ray photoelectron spectroscopy apparatus used is a PHI Quanera II manufactured by ULVAC-PHI (stock). [Tensile Strength of Carbon Fiber Substrate] Test pieces with a width of 50 mm and a length of 150 mm were cut from the carbon fiber substrate. 50 mm portions at each end were supported, and the remaining 50 mm portion in the center was subjected to a tensile test at room temperature and a test speed of 3 m / min. The tensile strength [N / cm] was obtained by dividing the maximum point load by the width of the test piece, and the arithmetic mean of five samples at each level was used. Furthermore, the tensile tests on the carbon fiber substrate were performed separately for levels that underwent a heat treatment at 300°C for 5 minutes and levels that did not undergo the heat treatment. Specifically, the tensile strength (Ta) of the level that underwent the heat treatment and the tensile strength (Tao) of the level that did not undergo the heat treatment were calculated as the tensile strength in a certain direction of the carbon fiber substrate. Furthermore, the tensile strength (Tb) of the level that underwent the heat treatment and the tensile strength (Tbo) of the level that did not undergo the heat treatment were calculated as the tensile strength in a direction orthogonal to the aforementioned direction. The change in tensile strength before and after heat treatment is calculated based on Ta and Tao using the following formula: (Tensile strength after heat treatment relative to tensile strength before heat treatment) = Ta / Tao × 100 [%] The tensile strength ratio after heat treatment (Ta / Tb) is calculated by dividing Ta by Tb, and the tensile strength ratio before heat treatment (Tao / Tbo) is calculated by dividing Tao by Tbo. [Thermogravimetric / mass analysis (TG-MS)] The mass and composition of the gas generated during heating were analyzed using a thermogravimetric / mass analysis apparatus (TG-MS). The rate of thermal mass reduction of the resin composition (B) was determined using thermogravimetric analysis (TG) by the following formula (1). Rate of thermal mass reduction (mass%) = [(W1-W2) / W1]×100…(1) Here, W1 is the mass (mg) of the sample after heating from room temperature to 100°C at 10°C / min and then holding it isothermally for 30 minutes, and W2 is the mass (mg) of the sample after heating to 250°C at 10°C / min. The GC / MSQP2010 (manufactured by Shimadzu Corporation) was used for the analysis of the generated gases by mass spectrometry (MS). The presence or absence of moisture in the generated gases was used to determine whether it was a dehydration condensation reaction or a thermal decomposition reaction. [Number-average fiber length of carbon fiber (A)] 400 carbon fibers were randomly selected from the carbon fiber nonwoven fabric used as carbon fiber (A), and their lengths were measured using an optical microscope. The number-average fiber length (Ln) was calculated using the following formula: Number-average fiber length (Ln) = (ΣLi) / Nf Li: Measured fiber length (i = 1, 2, 3, ..., n) Nf: Total number of fibers whose lengths were measured [Average out-of-plane expansion rate of the prepreg] The out-of-plane expansion rate of the prepreg is calculated based on the thickness of the prepreg and the thickness of the porous structure formed by heating the prepreg at a temperature of 20°C above the melting point of the matrix resin (C) for 5 minutes, using the following formula: Out-of-plane expansion rate of the prepreg (%) = [Thickness of the porous structure (mm) / Thickness of the prepreg (mm)] × 100. The average out-of-plane expansion rate of the prepreg is calculated as the arithmetic mean of the out-of-plane expansion rates of the prepreg, obtained by measurements at five randomly selected locations. [Density of prepreg or porous structure] Density of prepreg or porous structure (g / cm³) 3 The sample mass (g) is divided by the volume (cm³) calculated based on the sample's perimeter. 3 The value obtained is the arithmetic mean of the results measured in 5 randomly selected samples. [Average Pore Diameter of Prepreg or Porous Structure] The average pore diameter, as shown in Figure 4 (a schematic diagram of the surface of the porous structure), is determined based on the diameter of the largest circle tangent to the pores. The surface of the prepreg or porous structure is observed using a scanning electron microscope (SEM). The largest circle tangent to the pores is traced in the observed image, and the diameter of this circle is calculated as the pore diameter. The arithmetic mean of the lengths obtained from measuring this value at 50 randomly selected locations is the average pore diameter. [Flexural Strength of Porous Structures] Flexural strength was measured using an Instron 5565 universal testing machine (manufactured by Instron Japan, Ltd.) according to the three-point bending test standard JIS K7171 (2016). The ambient temperature was regulated using a thermostatic bath, and flexural strength was measured at various temperatures of 30°C and 140°C. The flexural strength was calculated as the arithmetic mean of five samples at each level. The retention rate of flexural strength at 140°C was then calculated using the following formula: (Retention rate of flexural strength at 140°C) = (Flexural strength at 140°C [MPa]) / (Flexural strength at 30°C [MPa]) × 100 [%). [Tensile Strength of Prepreg] The tensile strength of the prepreg was determined using an Instron 5565 universal testing machine (manufactured by Instron Japan) and according to the tensile test method of the International Organization for Standardization (ISO) 527-3 (1995). For the test pieces, test pieces were prepared by cutting along four directions: 0°, 45°, 90°, and -45°. For each direction, the number of measurements was set to n=5. For all measured values ​​(n=20), the maximum and minimum values, and the average value of these values ​​were calculated, which is the average tensile strength. The standard deviation was then calculated, and the CV value (coefficient of variation), which can be calculated using the following formula, was taken as the deviation of the tensile strength. The deviation of tensile strength (%) = [(standard deviation of tensile strength) / (average value of tensile strength)] × 100. Furthermore, by dividing the maximum value of tensile strength by the minimum value of tensile strength, the ratio of the maximum value of tensile strength to the minimum value is obtained. [Thickness and Deviation of Prepreg at 400°C] A heating plate including a laser displacement meter was used to measure thickness non-contactly. The prepreg was placed on the heating plate and heated to 400°C. The thickness of the prepreg was read after holding the temperature for 5 minutes. The thickness of the prepreg was read at 20 randomly selected locations, and the average value was taken as the thickness at 400°C. The standard deviation was then calculated, and the CV value (coefficient of variation), which can be calculated by the following formula, was taken as the thickness deviation at 400°C. Thickness deviation at 400°C (%) = [(Standard deviation of thickness at 400°C) / (Average thickness at 400°C)] × 100. [Coverage of Adhesive Resin Composition (D)] The coverage of the adhesive resin composition (D) on the carbon fiber substrate can be calculated using the surface oxygen concentration (O / C) obtained by X-ray photoelectron spectrometry and the following formula: Coverage of adhesive resin composition (D) (%) = {(O / C of carbon fiber (A)) - (O / C of carbon fiber substrate)} / {(O / C of carbon fiber (A)) - (O / C of adhesive resin composition (D))} × 100. In the samples used here, in addition to the carbon fiber substrate, carbon fiber (A) monomer before coating with adhesive resin composition (D) and the precursor of adhesive resin composition (D) were used to prepare a 0.1 mm thick film by heat treatment under the same conditions as when preparing the carbon fiber substrate. The surface oxygen concentration (O / C) of each sample was determined in the same manner as the "coverage of resin composition (B)". [Reduction in Thermal Mass] The reduction in thermal mass of carbon fiber (A) and binder resin composition (D) was determined using thermogravimetric analysis (TG) and the following formula: Reduction in thermal mass (mass%) = (M1 - M2) / M1 × 100 Here, M1 is the mass (mg) of the sample after isothermal holding at 30°C for 10 minutes, and M2 is the mass (mg) of the sample after heating at 10°C / min to reach 400°C. The carbon fiber (A) sample used was the carbon fiber (A) before coating with binder resin composition (D). For the binder resin composition (D) sample, the precursor of binder resin composition (D) obtained by heat treatment separately under the same conditions as when making the carbon fiber substrate was used for evaluation. [Glass transition temperature of adhesive resin composition (D)] The glass transition temperature of adhesive resin composition (D) was determined using differential scanning calorimetry (DSC) according to JIS K7121 (2012). For the samples of adhesive resin composition (D), those obtained by separately heating the precursor of adhesive resin composition (D) under the same conditions as those used in the fabrication of the carbon fiber substrate were used for evaluation. [Fiber Length of Carbon Fiber (A)] 400 carbon fibers (A) were randomly selected from the carbon fiber substrate, and their lengths were measured using an optical microscope. The number average fiber length (Ln) was calculated using the following formula: Number average fiber length (Ln) = (ΣLi) / Nf Li: Measured fiber length (i = 1, 2, 3, ..., n) Nf: Total number of fibers whose lengths were measured [Mass Ratio of Carbon Fiber (A) in Carbon Fiber Substrate] The mass ratio of carbon fiber (A) in carbon fiber substrate is calculated based on the mass of carbon fiber substrate and the mass of carbon fiber (A) before coating with the adhesive resin composition, using the following formula: Mass ratio of carbon fiber (A) in carbon fiber substrate (%) = (Mass of carbon fiber (A) (g)) / (Mass of carbon fiber substrate (g)) × 100. [Mass percentage of carbon fiber (A) in the prepreg] The mass percentage of carbon fiber (A) in the prepreg is calculated based on the mass of the prepreg and the mass of carbon fiber (A) used therein, using the following formula: Mass percentage of carbon fiber (A) in the prepreg (%) = (Mass of carbon fiber (A) (g)) / (Mass of prepreg (g)) × 100. [Expansion Rate of Fiber-Reinforced Resin Molded Body or Void Body] The expansion rate of the fiber-reinforced resin molded body or void body is calculated based on the thickness of the preform used in molding and the thickness of the fiber-reinforced resin molded body or void body obtained by molding the preform, using the following formula: (Expansion Rate) = (Thickness of Fiber-Reinforced Resin Molded Body or Void Body [mm]) / (Thickness of Preform [mm]) × 100 [%] The thickness of the fiber-reinforced resin molded body or void body and the thickness of the preform are calculated as the average of the thicknesses measured at 5 randomly selected locations. [Density of fiber-reinforced resin molded articles or voids] Density of fiber-reinforced resin molded articles or voids [g / cm³] 3 [ ] is the sample mass [g] divided by the volume [cm³] calculated based on the sample's perimeter. 3 The value obtained is the average of the results measured in 5 randomly selected samples. The carbon fiber substrate, prepreg, and porous structure prepared in the embodiments and comparative examples of the present invention will be described below. [Example 1] (Carbon fiber (A)) As carbon fiber (A), carbon fiber nonwoven fabric CF-1 was cut into 300 mm × 300 mm size. (Resin Composition (B)) PAMC was used as resin composition (B). It was prepared and then diluted to a 1% by mass aqueous solution. (Carbon fiber substrate) The carbon fiber nonwoven fabric is immersed in the aqueous solution. Then, the carbon fiber nonwoven fabric is removed and dried at 100°C for 1 hour, thereby producing a carbon fiber substrate. The difference between the mass of the obtained carbon fiber substrate and the mass of the carbon fiber (A) used is the amount of resin composition (B) attached, which is 5 parts by mass relative to 100 parts by mass of carbon fiber (A). Furthermore, the coverage rate of resin composition (B) is 100%. The results of each evaluation are shown in Table 1. [Example 2] The concentration of the resin composition (B) was set to 3% by mass when it was further diluted, thereby setting the amount of resin composition (B) attached to 10 parts by mass. Otherwise, the carbon fiber substrate was obtained in the same manner as in Example 1. The resin composition (B) in the carbon fiber substrate has a 100% coverage. The results of each evaluation are shown in Table 1. [Comparative Example 1] A carbon fiber substrate was obtained in the same manner as in Example 1, except that PVA was used as the resin component (B). The PVA coverage of this sample was 100%. The results of each evaluation are shown in Table 1. [Comparative Example 2] Carbon fiber nonwoven fabric CF-1 was used directly for evaluation without using resin composition (B). The results of each evaluation are shown in Table 1. [Example 3] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as used in Example 1 was used. (Resin Composition (B)) PAA was used as resin composition (B). It was prepared into a 1% by mass solution of N-methyl-2-pyrrolidone (NMP). (Carbon fiber substrate) The carbon fiber nonwoven fabric is immersed in the NMP solution. Then, the carbon fiber nonwoven fabric is removed from the NMP solution and immersed in water, thereby precipitating resin composition (B) on the surface of the carbon fibers (A). The carbon fiber nonwoven fabric is then removed from the water and dried at 100°C for 1 hour, thereby producing the carbon fiber substrate. The amount of resin composition (B) attached is 5 parts by mass relative to 100 parts by mass of carbon fiber (A). Furthermore, the coverage rate of resin composition (B) is 100%. The results of each evaluation are shown in Table 1. [Table 1] [Examples 1-2] (Carbon fiber substrate) The carbon fiber substrate obtained in Example 1 was used as the carbon fiber substrate. (Matrix Resin (C)) As matrix resin (C), two sheets containing PEKK and with a unit area weight of 150 g / m² were used. 2 A 300 mm × 300 mm resin film. (Prepreg) The carbon fiber substrate and resin film are laminated in the order of resin film / carbon fiber substrate / resin film. Then, the laminate is clamped using a 400 mm × 400 mm steel use stainless steel (SUS) flat mold (tooling plate), thereby heating and pressing the laminate while it is being transported. The pressing step is first performed by heating at 350°C and 10 MPa for 10 minutes. Then, it is cooled and cured at 150°C and 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm. Regarding the appearance of the obtained prepregs, no damage was found in the carbon fiber substrate, and no molding defects were identified. The results of each evaluation are shown in Table 2. [Examples 2-2] Prepregs were prepared in the same manner as in Examples 1-2, except that the carbon fiber substrate obtained in Example 2 was used as the carbon fiber substrate. Regarding the appearance of the obtained prepregs, no damage was found in the carbon fiber substrate, and no molding defects were identified. The results of each evaluation are shown in Table 2. [Comparative Examples 1-2] The prepregs were prepared in the same manner as in Examples 1-2, except that the carbon fiber substrate obtained in Comparative Example 1 was used as the carbon fiber substrate. In the obtained prepregs, poor forming due to damage to the carbon fiber substrate was found in some locations. The results of the evaluations conducted at locations with less damage are shown in Table 2. [Comparative Examples 2-2] Prepregs were attempted to be made in the same manner as in Examples 1-2, except that the carbon fiber nonwoven fabric CF-1 evaluated in Comparative Example 2 was used as the carbon fiber substrate. However, the carbon fiber substrate broke and fractured during compression molding, and a prepreg could not be obtained. The results are shown in Table 2. [Example 3-2] The prepreg was made in the same manner as in Example 1-2, except that the carbon fiber substrate obtained in Example 3 was used as the carbon fiber substrate. Regarding the appearance of the obtained prepregs, no damage was found in the carbon fiber substrate, and no molding defects were identified. The results of each evaluation are shown in Table 2. [Table 2] [Examples 1-3] (Prepreg) The prepreg obtained in Examples 1-2 was used as the prepreg. (Porous Structure) Four 300 mm × 300 mm prepreg volumes with a thickness of 0.28 mm were used and clamped together with 4 mm thick SUS spacers between 400 mm × 400 mm SUS flat molds (tooling plates), thereby controlling the expansion ratio while heating and pressing. In the pressing step, heating was first performed at 350°C and 1 MPa for 10 minutes, followed by cooling and curing at 150°C and 1 MPa for 10 minutes. The expansion ratio was controlled at 360%, and the thickness of the resulting porous structure was 4 mm. The evaluation results are shown in Table 3. [Comparative Examples 1-3] Porous structures were fabricated in the same manner as in Examples 1-3, except that the prepreg obtained in Comparative Examples 1-2 was used as the prepreg. The results of each evaluation are shown in Table 3. [Examples 3-3] Porous structures were fabricated in the same manner as in Examples 1-3, except that the prepreg obtained in Example 3-2 was used as the prepreg. The results of each evaluation are shown in Table 3. [Examples 1-4] (Prepreg) The prepreg obtained in Examples 1-2 was used as the prepreg. (Porous Structure) Three 300 mm × 300 mm prepreg volumes with a thickness of 0.28 mm were used and clamped together with 4 mm thick SUS spacers between 400 mm × 400 mm SUS flat molds (tooling plates), thereby controlling the expansion ratio while heating and pressing. In the pressing step, heating was first performed at 350°C and 1 MPa for 10 minutes, followed by cooling and curing at 150°C and 1 MPa for 10 minutes. The expansion ratio was controlled at 480%, and the thickness of the resulting porous structure was 4 mm. The evaluation results are shown in Table 3. [Comparative Examples 1-4] Porous structures were attempted to be fabricated in the same manner as in Examples 1-4, except that the prepreg obtained in Comparative Examples 1-2 was used as the prepreg. However, the out-of-plane expansion ratio was insufficient, and the target 4 mm thickness porous structure could not be obtained. The results of each evaluation are shown in Table 3. [Examples 3-4] Porous structures were fabricated in the same manner as in Examples 1-4, except that the prepreg obtained in Examples 3-2 was used as the prepreg. The results of each evaluation are shown in Table 3. [Table 3] [Example 4] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as used in Example 1 was used. (Resin Composition (B)) The solution of resin composition (B), which is the precursor of adhesive resin composition (D), was prepared using a 1% aqueous solution of PAMC, the same as the solution used in Example 1. (Carbon fiber substrate) The carbon fiber nonwoven fabric is immersed in the aqueous solution. Then, the carbon fiber nonwoven fabric is removed and heated and dried at 200°C for 1 hour to amide-imide the PAMC, thereby obtaining a carbon fiber substrate formed by attaching polyether amide resin (D) to carbon fiber (A). The reinforcing fiber (A) has a fiber length of 6 mm, and the coverage of the adhesive resin composition (D) is 100%. The glass transition temperature of the adhesive resin composition (D) is 220°C, and the thermal mass reduction rate at 400°C is 1% by mass. (Matrix Resin (C)) As matrix resin (C), two sheets containing PEKK and with a unit area weight of 150 g / m² were used. 2 A 300 mm × 300 mm resin film. (Prepreg) The carbon fiber substrate and resin film are laminated in the order of resin film / carbon fiber substrate / resin film. Then, the laminate is clamped using a 400 mm × 400 mm SUS flat mold (tooling plate), thereby heating and pressing the laminate while it is being transported. In the pressing step, it is first heated at 350°C and 10 MPa for 10 minutes, and then cooled and cured at 30°C and 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm. The evaluation results are shown in Table 4. [Example 5] The prepreg was obtained in the same manner as in Example 4, except that the concentration of the PAMC aqueous solution was set to 3% by mass. The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the adhesive resin composition (D) was 100%. The glass transition temperature of the adhesive resin composition (D) was 220°C, and the thermal mass reduction rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Example 6] The prepreg was obtained in the same manner as in Example 4, except that CF-2 was used as the carbon fiber (A). The fiber length of the carbon fiber (A) was 3 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 220°C, and the thermal mass reduction rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Example 7] The prepreg was obtained in the same manner as in Example 4, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the adhesive resin composition (D) was 100%. The glass transition temperature of the adhesive resin composition (D) was 220°C, and the thermal mass reduction rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Example 8] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as used in Example 1 was used. (Resin Composition (B)) As a precursor to adhesive resin composition (D), resin composition (B) used PAA. It was prepared and then diluted to a 1% by mass NMP solution. (Carbon fiber substrate) The carbon fiber nonwoven fabric is impregnated in the NMP solution. Subsequently, the carbon fiber nonwoven fabric is removed and heated and dried at 200°C for 1 hour to imidize the PAA, thereby obtaining a carbon fiber substrate formed by attaching polybenzimidazole resin (as adhesive resin composition (D)) to carbon fiber (A). The coverage of adhesive resin composition (D) is 100%. The glass transition temperature of adhesive resin composition (D) is 425°C, and the thermal mass reduction rate at 400°C is 1% by mass. (Matrix Resin (C)) As matrix resin (C), two sheets containing PEKK and with a unit area weight of 150 g / m² were used. 2 A 300 mm × 300 mm resin film. (Prepreg) The carbon fiber substrate and resin film are laminated in the order of resin film / carbon fiber substrate / resin film. Then, the laminate is clamped using a 400 mm × 400 mm SUS flat mold (tooling plate), thereby heating and pressing the laminate while it is being transported. In the pressing step, it is first heated at 350°C and 10 MPa for 10 minutes, and then cooled and cured at 30°C and 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm. The evaluation results are shown in Table 4. [Example 9] The prepreg was obtained in the same manner as in Example 8, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the adhesive resin composition (D) was 100%. The glass transition temperature of the adhesive resin composition (D) was 425°C, and the thermal mass reduction rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Comparative Example 3] The prepreg was obtained in the same manner as in Example 4, except that PVA was used as the resin composition (B). The coverage of the adhesive resin composition (D) was 100%. The glass transfer temperature of the adhesive resin composition (D) was 60°C, and the thermal mass reduction rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Comparative Example 4] The prepreg was obtained in the same manner as in Comparative Example 3, except that CF-2 was used as the carbon fiber (A). The fiber length of the carbon fiber (A) was 3 mm, and the coverage of the adhesive resin composition (D) was 100%. The glass transition temperature of the adhesive resin composition (D) was 60°C, and the thermal mass reduction rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Comparative Example 5] The prepreg was obtained in the same manner as in Comparative Example 3, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the adhesive resin composition (D) was 100%. The glass transition temperature of the adhesive resin composition (D) was 60°C, and the thermal mass reduction rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4. [Table 4] [Example 10] (Prepreg) The prepreg obtained in Example 4 was used as the prepreg. (Preform) Four prepreg volume layers of 300 mm × 300 mm and 0.28 mm thickness are used to prepare a preform. (Fiber-reinforced resin molded body) The preform was clamped between two 400 mm × 400 mm SUS flat molds (tooling plates) and subjected to heating and pressure pressing. In the pressing step, it was first heated at 350°C and 3 MPa for 10 minutes, followed by cooling and curing at 30°C and 5 MPa for 10 minutes. The resulting fiber-reinforced resin molded body had a thickness of 1.12 mm, an expansion rate of 100%, and a density of 1.4 g / cm³. 3 It is a molded product without gaps. [Example 11] (Prepreg) The prepreg obtained in Example 4 was used as the prepreg. (Preform) Four prepreg volume layers of 300 mm × 300 mm and 0.28 mm thickness are used to prepare a preform. (Fiber-reinforced resin molded body) The preform, together with a 4 mm thick SUS spacer, is clamped between a 400 mm × 400 mm SUS flat mold (tooling plate), thereby controlling the expansion rate while heating and pressing. In the pressing step, it is first heated at 350°C and 3 MPa for 10 minutes, then cooled and cured at 30°C and 1 MPa for 10 minutes to obtain a porous fiber-reinforced resin molded body (void body). The obtained void body has a thickness of 4 mm, an expansion rate of 360%, and a density of 0.4 g / cm³. 3 The bending strength at 30℃ is 53 MPa, the bending strength at 140℃ is 48 MPa, and the retention rate of bending strength when heated to 140℃ is 91%. [Example 12] A void body was formed in the same manner as in Example 11, except that the prepreg obtained in Example 8 was used as the prepreg. The obtained void body had a thickness of 4 mm, an expansion rate of 360%, and a density of 0.4 g / cm³. 3 The bending strength at 30℃ is 53 MPa, the bending strength at 140℃ is 50 MPa, and the retention rate of bending strength when heated to 140℃ is 94%. [Comparative Example 6] Except that the prepreg obtained in Comparative Example 3 was used as the prepreg, a void body was formed in the same manner as in Example 11. The obtained void body had a thickness of 4 mm, an expansion rate of 360%, and a density of 0.4 g / cm³. 3 The bending strength at 30℃ is 47 MPa, the bending strength at 140℃ is 32 MPa, and the bending strength retention rate when heated to 140℃ is 68%. [Example 13] (Prepreg) The prepreg obtained in Examples 1-2 was used as the prepreg. (Preform) Two prepreg volume layers, each 300 mm long, 300 mm wide, and 0.28 mm thick, are used to prepare a preform. (Fiber-reinforced resin molded body) The preform is clamped between a pair of molds with concave and convex shapes, and heated and pressed while controlling the expansion rate. In the pressing step, it is first heated at 350°C and 3 MPa for 10 minutes, and then cooled and cured at 30°C and 1 MPa for 10 minutes. The obtained fiber-reinforced resin molded body has a thickness of 2 mm, an expansion rate of 360%, and a density of 0.4 g / cm³. 3 Furthermore, the obtained fiber-reinforced resin molded body is formed into a corrugated shape by the concave and convex shape of the transfer mold. When viewed from one of the surfaces, it is a corrugated plate shape in which the fiber-reinforced resin molded body, which is 10 mm long in the length direction, is repeatedly bent in the order of bending angle 120° / 120° / 240° / 240°. [Example 14] The corrugated fiber-reinforced resin molded body obtained in Example 13 was cut into strips with a width of 10 mm. The strips were laminated with the protrusions of adjacent strips facing each other, thereby joining the protrusions together. An epoxy resin adhesive (120°C curing type) was applied to the joint surfaces of the protrusions, and the molded body was heated and pressurized at 120°C and 0.1 MPa for 1 hour to obtain a honeycomb structure fiber-reinforced resin molded body with a honeycomb structure consisting of hollow columnar cells arranged in a regular hexagonal prism shape with one side of 10 mm. [Example 15] (Core Material) The honeycomb-shaped fiber-reinforced resin molded body obtained in Example 14 was used as the core material. (Sheet material) Two pieces of prepreg obtained in Examples 1-2 were used as a pair of sheet materials. (Sandwich Structure) An epoxy resin adhesive (120°C curing type) is applied to each of the opposing surfaces of the two outer skin materials. The core material is sandwiched between the outer skin materials and heated and pressurized at 120°C and 0.1 MPa for 1 hour to integrate them and form a sandwich structure. 1: Carbon fiber (A) 2: Resin composition (B) or adhesive resin composition 3: Pores 4: Carbon fiber (A) coated with resin composition (B) or adhesive resin composition 5: Matrix resin (C) 6: In-plane orientation 7: Out-of-plane orientation 8: Pore diameter (diameter of the inscribed circle of the pore) Figure 1 is a schematic diagram showing an embodiment of the carbon fiber substrate of the present invention. The right-hand side is an enlarged view showing the state of the carbon fiber (A) and the resin composition (B) attached to its surface. Figure 2 is a schematic diagram showing an embodiment of the prepreg of the present invention and an enlarged view of its internal cross-section. Figure 3 is a schematic diagram showing an embodiment of the porous structure of the present invention and an enlarged view of its internal cross-section. Figure 4 is a schematic diagram illustrating the pore diameter of the porous structure. 1: Carbon fiber (A) 2: Resin composition (B) or adhesive resin composition 3: Porosity 4: Carbon fibers (A) coated with resin composition (B) or adhesive resin composition.

Claims

1. A carbon fiber substrate comprising carbon fibers (A) and a resin composition (B) covering the surface of the carbon fibers (A), wherein the carbon fibers (A) are formed into a nonwoven fabric, wherein the carbon fiber substrate has tensile strength Ta, measured at room temperature after heating at 300°C for 5 minutes as tensile strength in a certain direction, and tensile strength Tao, measured at room temperature without heat treatment, and tensile strength Tb, measured at room temperature after heating at 300°C for 5 minutes as tensile strength in a direction orthogonal to the certain direction, and tensile strength Tbo, measured at room temperature without heat treatment, wherein Ta and Tb are 25 N / cm or more and 100 N / cm or less, and Tao and Tbo are 8 N / cm or more and 25 N / cm or less, and the tensile strength of the carbon fiber substrate after heating at 300°C for 5 minutes is 300% or more of the tensile strength before heating.

2. The carbon fiber substrate as claimed in claim 1, wherein, The resin composition (B) contains at least one selected from polyamide, polyamide salt, polyamide ester, polyhydroxyamide and polyaminoamide.

3. The carbon fiber substrate as described in claim 1 or 2, wherein, The resin composition (B) is capable of undergoing a dehydration condensation reaction, wherein the heat mass reduction rate defined by the following formula (1) in the reaction is 5% to 50% by mass, heat mass reduction rate (mass%) = [(W1-W2) / W1]×100…(1) Here, W1 is the sample mass (mg) after heating from room temperature to 100°C at 10°C / min and then holding it isothermally for 30 minutes, and W2 is the sample mass (mg) after heating at 10°C / min to reach 250°C.

4. The carbon fiber substrate as described in claim 1 or 2, wherein, The ratio of tensile strength (Ta / Tb) obtained by dividing the tensile strength (Ta) in any direction of the carbon fiber substrate measured at room temperature after heating at 300°C for 5 minutes by the tensile strength (Tb) in its orthogonal direction is 0.6 to 1.

7.

5. The carbon fiber substrate as claimed in claim 1 or 2, wherein, The average fiber length of carbon fiber (A) ranges from 0.1 mm to 100 mm.

6. The carbon fiber substrate as claimed in claim 1 or 2, wherein, The resin composition (B) is 0.5 to 30 parts by mass relative to 100 parts by mass of carbon fiber (A).

7. The carbon fiber substrate as claimed in claim 1 or 2, wherein, The carbon fiber (A) in the carbon fiber substrate has a mass ratio of more than 90% and less than 97%.

8. A prepreg made by impregnating a matrix resin (C) in a carbon fiber substrate as described in any one of claims 1 to 7, or a substrate derived from a carbon fiber substrate as described in any one of claims 1 to 7, wherein the amount of the matrix resin (C) is 5 to 1,000 parts by mass relative to 100 parts by mass of the carbon fiber (A), and the mass percentage of the carbon fiber (A) in the prepreg is 1% or more and 50% or less.

9. The prepreg as claimed in claim 8 is formed by impregnating a matrix resin (C) in a substrate comprising carbon fiber (A) and an adhesive resin composition, wherein the adhesive resin composition is a resin composition (B) or derived from the resin composition (B), the glass transition temperature of the adhesive resin composition is 120°C or higher and 450°C or lower, the melting point of the matrix resin (C) is 250°C or higher and 400°C or lower, the adhesive resin composition is present at the interface between the carbon fiber (A) and the matrix resin (C), and the prepreg satisfies the following conditions (i) and (ii): (i) the minimum tensile strength is 100 MPa or higher; (ii) the thickness deviation at 400°C is 10% or lower.

10. A prepreg comprising impregnating a matrix resin (C) into a substrate comprising carbon fibers (A) and an adhesive resin composition (D), wherein, Carbon fiber (A) is a discontinuous fiber and forms a nonwoven fabric. The glass transition temperature of the adhesive resin composition (D) is 120°C or higher and 450°C or lower. The tensile strength of the substrate in a certain direction is measured at room temperature after heating at 300°C for 5 minutes, and at room temperature after no heat treatment, Ta and Tb are measured at room temperature after heating at 300°C for 5 minutes, and at room temperature after no heat treatment, Tbo are measured at room temperature after no heat treatment, for the substrate in a direction orthogonal to the stated direction. Ta and Tb are 25 N / cm or higher and 100 N / cm or lower, and Tao and Tbo are 8 N / cm or higher and 25 N / cm or lower. The tensile strength of the substrate after heating at 300°C for 5 minutes is 300% or higher than that before heating. The melting point of the matrix resin (C) is 250°C or higher and 400°C or lower. The adhesive resin composition (D) is present at the interface between the carbon fiber (A) and the matrix resin (C). The amount of matrix resin (C) is 5 to 1,000 parts by mass relative to 100 parts by mass of carbon fiber (A), the mass ratio of carbon fiber (A) in the prepreg is 1% or more and 50% or less, and the prepreg satisfies the following conditions (i) and (ii): (i) the minimum tensile strength is 100 MPa or more; (ii) the thickness deviation at 400°C is 10% or less.

11. The prepreg as claimed in claim 10, wherein, The adhesive resin composition (D) is a composition comprising a polymer or a derivative thereof having at least one structure selected from the following polymers: etherimide skeleton, benzoxazole skeleton, benzimidazole skeleton, benzoxazine skeleton and cyanate ester skeleton.

12. The prepreg as described in claim 10 or 11, wherein, The adhesive resin composition (D) comprises at least one resin selected from phenolic resin, urea resin, melamine resin, polyimide resin, polybenzoxazole resin, polybenzimidazole resin, bismaleimide resin, benzoxazine resin, and cyanate ester resin.

13. The prepreg as described in claim 10 or 11, wherein, The adhesive resin composition (D) has a thermal mass reduction rate of less than 5% by mass at 400°C.

14. The prepreg as claimed in any one of claims 8 to 11, wherein, The matrix resin (C) contains at least one selected from polyetherketone, polyetheretherketone, and polyetherketoneketone as a main component.

15. The prepreg as claimed in any one of claims 8 to 11, wherein, The carbon fiber (A) comprises more than 50% by mass of fibers with a length of more than 2 mm and less than 10 mm.

16. The prepreg as claimed in any one of claims 8 to 11, wherein, The thickness at 400℃ is 0.1 mm or more and 5 mm or less.

17. The prepreg as claimed in any one of claims 8 to 11, wherein, The deviation of the in-plane tensile strength is less than 20%.

18. The prepreg as claimed in any one of claims 8 to 11, wherein, The maximum tensile strength is above 260 MPa.

19. The prepreg as claimed in any one of claims 8 to 11, wherein, The ratio of the maximum tensile strength to the minimum tensile strength is 1 to 3.

20. The prepreg as claimed in any one of claims 8 to 11, wherein, When the matrix resin (C) melts, it expands with an average out-of-plane expansion rate of 150% to 1,000%.

21. A porous structure formed from a prepreg as described in any one of claims 8 to 20.

22. The porous structure as claimed in claim 21, wherein, The bending strength at 140°C is more than 80% of the bending strength at 30°C.

23. A method for manufacturing a porous structure, comprising: The first step involves melting and expanding the matrix resin (C); The second step involves curing the matrix resin (C).

24. A preform comprising a prepreg as a lamination unit as claimed in any one of claims 8 to 20.

25. A fiber-reinforced resin molded article, formed by heating and pressurizing a preform as described in claim 24.

26. A fiber-reinforced resin molded article, formed by heating and expanding a preform as described in claim 24, and having voids.

27. The fiber-reinforced resin molded article as described in claim 25 or 26 is corrugated or honeycomb shaped.

28. A sandwich structure comprising sandwiching and integrally forming a fiber-reinforced resin molded body as described in any one of claims 25 to 27 between a pair of skin materials as a core material.

29. An aircraft component comprising a prepreg as described in any one of claims 8 to 20, a porous structure as described in claims 21 or 22, a fiber-reinforced resin molded body as described in any one of claims 25 to 27, or a sandwich structure as described in claim 28.

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