JP6750815B2 - Alumina fiber, alumina fiber sheet, alumina fiber-organic resin composite sheet, and method for producing alumina fiber - Google Patents

Description

The present invention relates to an alumina fiber, an alumina fiber sheet, an alumina fiber-organic resin composite sheet, and an alumina fiber manufacturing method. The alumina fiber-organic resin composite sheet of the present invention can be used for applications requiring thermal conductivity, such as semiconductor device applications, thermal printer applications, adhesive applications, and solar cell applications.

For example, in order to efficiently dissipate heat generated from a semiconductor element or the like, a resin is compounded with a heat conductive filler. As the filler, those having a particle shape, a plate shape, or a fibrous shape are known. Among these fillers, a widely used method for obtaining high thermal conductivity is a method in which a particulate filler is uniformly dispersed in a resin and highly filled. Thus, by uniformly dispersing and highly filling, percolation (heat transfer path) can be secured and heat can be efficiently dissipated.

For example, Japanese Unexamined Patent Application Publication No. 2012-033638 (Patent Document 1) describes, as a film capable of efficiently dissipating heat, "a flip-chip type semiconductor provided on the back surface of a semiconductor element flip-chip connected to an adherend. A film for the back surface, containing a resin and a heat conductive filler, the content of the heat conductive filler is 50% by volume or more, the average particle diameter of the heat conductive filler with respect to the thickness of the film. A film for a flip-chip type semiconductor back surface, which has a value of 30% or less and a maximum particle size of 80% or less." However, since it contains a large amount of the thermally conductive filler of 50% by volume or more, the viscosity of the resin increases, the thermally conductive filler aggregates, the strength of the film decreases, and the moldability deteriorates. There were problems such as.

On the other hand, by combining the fibrous filler having a high aspect ratio with the resin as compared with the particulate filler, the percolation can be secured with a smaller content than when the particulate or plate-like filler is compounded. It is possible to obtain a film that can efficiently dissipate heat without impairing the strength and moldability of the film.

As an alumina fiber that is considered to be usable as such a thermally conductive fibrous filler, "a spinning solution prepared by dispersing boehmite particles in a polyvinyl alcohol (PVA) aqueous solution is electrostatically spun to form a PVA-boehmite nanocomposite. After the formation, a heat-treated alumina nanofiber was produced” (Non-Patent Document 1). However, since alumina nanofibers produced by baking and solidifying particles such as boehmite particles have the effect of grain boundaries, even when used as fillers, the alumina nanofibers are crushed by the pressure and shearing force applied during molding. However, the effect of improving mechanical properties or thermal conductivity as a filler cannot be sufficiently exhibited.

In addition, Japanese Patent Laid-Open No. 2015-086270 (Patent Document 2) aims at improving the thermal conductivity of a “filler-dispersed organic resin composite in which fibrous alumina filler is dispersed in an organic resin, The filler-dispersed organic resin composite is disclosed in which the organic resin composite has a thermal conductivity of 3 W/m·K or more at a thickness of 0.3 mm. However, although the fibrous alumina filler (alumina fiber) described in the above publication is excellent in mechanical strength and thermal conductivity, it was not designed in consideration of flexibility. Further, there has been a long-awaited demand for alumina fibers having higher thermal conductivity.

JP 2012-033638 A JP, 2005-086270, A

Koji Nakane, 3 others, “Formation and use of alumina nanofibers made from organic-inorganic composites as precursors”, Journal of functional paper, October 2012

Under such circumstances, an object of the present invention is to provide an alumina fiber, an alumina fiber sheet, an alumina fiber-organic resin composite sheet and a method for producing an alumina fiber, which are excellent in strength and flexibility.

The present invention is
[1] An alumina fiber composed of 95 vol% or more of α-alumina, wherein the α-alumina has a crystallite size of 500 Å or less,
[2] A fiber sheet made of the alumina fiber according to the above [1], which has a bending strength of 5 MPa or more,
[3] A fiber sheet made of the alumina fiber according to the above [1], which has a bending deflection amount of 0.6 mm or more,
[4] An alumina fiber-organic resin composite sheet in which the alumina fiber according to the above [1] is dispersed in an organic resin,
[5] An alumina fiber-organic resin composite sheet containing the alumina fiber sheet according to the above [2] or [3] in an organic resin,
[6] (i) A step of spinning a spinnable sol solution prepared by hydrolyzing a solution containing an aluminum alkoxide at a temperature of 5° C. or lower, and then polycondensing the solution.
(Ii) a step of sintering the fiber spun in the step (i) at a temperature of 1100° C. or lower,
The present invention relates to a method for producing an alumina fiber.

The alumina fiber [1] is made of α-alumina of 95 vol% or more, and has a small crystallite size of α-alumina of 500 Å or less, which is small in grain boundary, and therefore is excellent in strength, flexibility and thermal conductivity.

Since the alumina fiber sheet [2] is a fiber sheet made of the alumina fiber [1], it has excellent bending strength of 5 MPa or more.

Since the alumina fiber sheet [3] is a fiber sheet made of the alumina fiber [1], it has excellent flexibility with a bending deflection amount of 0.6 mm or more.

The alumina fiber-organic resin composite sheet of [4] (hereinafter, may be simply referred to as a composite sheet) has strength, flexibility and thermal conductivity because the alumina fibers are dispersed in the organic resin. It is an excellent composite sheet.

The alumina fiber-organic resin composite sheet of [5] is a composite sheet that is excellent in strength, flexibility and thermal conductivity because it contains the alumina fiber sheet in the organic resin.

In the method for producing an alumina fiber according to the above [6], (i) a solution containing an aluminum alkoxide is hydrolyzed at a temperature of 5° C. or lower to suppress polycondensation during hydrolysis and form a three-dimensional structure. Since it is easy to form a one-dimensional structure by suppressing the above, the α-alumina fiber can be produced by sintering at a low temperature of 1100° C. or lower. In this way, α-alumina can be formed by sintering at a temperature as low as 1100° C. or lower, and the growth of α-alumina crystallites can be suppressed. Alumina fibers having excellent strength, flexibility and thermal conductivity can be produced.

It is a top view which shows typically the arrangement|positioning state of the alumina fiber in the alumina fiber sheet formed by the electrostatic spinning method. It is a top view which shows typically the arrangement|positioning state of the alumina fiber in the alumina fiber sheet formed by methods other than the electrostatic spinning method. It is a schematic cross section of the electrostatic spinning device used in Example 1 and Comparative Examples 1-3.

(Alumina fiber of the present invention)
The alumina fiber of the present invention is an alumina fiber composed of 95 vol% or more of α-alumina, and the crystallite size of the α-alumina is 500 Å or less.
The α-alumina ratio in the alumina fiber of the present invention is more preferably 97 vol% or more, further preferably 99 vol% or more, and most preferably 100 vol% so as to have excellent thermal conductivity.
Further, the crystallite size of α-alumina in the alumina fiber of the present invention is more preferably 450 Å or less, and further preferably 400 Å or less so that it has excellent strength, flexibility and thermal conductivity.

The “α-alumina ratio” in the present specification is a result of measurement using an X-ray diffractometer (for example, a desktop X-ray diffractometer MiniFlex600, manufactured by Rigaku Corporation) (measurement condition: 2θ measurement range +3 to 140°, tube Voltage: 30 kV, tube current: 15 mA) means a value quantified using a reference intensity ratio (RIR).
The “crystallite size of α-alumina” in the present specification means the crystallite size obtained from the half width using the Scherrer method based on the measurement results obtained under the above-mentioned measuring device and measuring conditions. In other words, the crystallite size is obtained by substituting the half-value width of the (116) plane, which is the strongest and most uniform in α-alumina, into the Scherrer's equation shown below. LaB ₆ is used as the standard substance.
Scherrer's formula: D=K×λ/(β×cos θ)
D: Crystallite size (Å)
K: Scherrer constant (K=0.94)
λ: Measured X-ray wavelength (Å)
β: Spread of diffraction line (rad) depending on crystallite size
θ: Bragg angle of diffraction line

When the composite sheet is manufactured using the alumina fiber of the present invention, when the alumina fiber is thin, it is easy to manufacture a composite sheet having a small thickness, and it is difficult for the composite sheet to project from the composite sheet. Therefore, the alumina fiber is preferably thin.

For example, when the average fiber diameter of the alumina fibers is 3 μm or less and is thin to some extent, it is easy to manufacture a thin composite sheet having uniform physical properties (mechanical strength and thermal conductivity). The smaller the average fiber diameter of the alumina fibers, the easier it is to manufacture a composite sheet that is thin and has uniform physical properties, and it is easy to respond to the recent trend toward lighter, thinner, shorter, and smaller products. Therefore, when manufacturing a composite sheet, the average fiber diameter is It is more preferably 2 μm or less, still more preferably 1 μm or less. On the other hand, the lower limit of the average fiber diameter is not particularly limited, but is appropriately about 0.01 μm, and preferably 0.05 μm or more. When the composite sheet is not used and the alumina fiber alone or the alumina fiber sheet is used, the average fiber diameter of the alumina fibers is not particularly limited.

The "average fiber diameter" in the present invention refers to the arithmetic average value of the fiber diameters at 50 points of the alumina fibers, and the "fiber diameter" is the length of the alumina fibers measured based on a 5000 times electron micrograph of the alumina fibers. The length in the direction orthogonal to the depth direction.

Further, the alumina fiber of the present invention may be substantially continuous fiber so as to have excellent thermal conductivity, or may be short fiber that is easily dispersed uniformly in a composite sheet. In addition, when the alumina fibers are “substantially continuous fibers”, when the alumina fibers are substantially continuous fibers, the alumina fibers are in a fiber sheet state in which the alumina fibers are aggregated, and thus the outer edge portion of the alumina fiber sheet is included. Set an arbitrary area to be defined below, take an electron micrograph at a magnification that can recognize the alumina fiber for a part of the area, and similarly recognize the alumina fiber for the other part of the area. Repeatedly taking electron micrographs at a magnification that can be taken, taking electron micrographs that cover only the entire area of the above region, observing these electron micrographs, and checking both ends of the alumina fiber. It means that it cannot be confirmed. That is, if one end can be observed but both ends cannot be observed, it is considered to be a substantially continuous fiber.
(Arbitrary area);
A square area having a side length equal to the fiber length of which the aspect ratio [=(average fiber length)/(average fiber diameter)] of the alumina fibers is 1500. For example, when the average fiber diameter is 3 μm, a square region having a side length of 4500 μm.

When the alumina fibers are short fibers as in the latter case, the aspect ratio is preferably 1000 or less. This is because when such an aspect ratio is used, when the composite sheet is manufactured, the alumina fibers are less likely to aggregate, and the composite sheet in which the alumina fibers are uniformly dispersed is easily manufactured. Considering the dispersibility of the alumina fiber, the aspect ratio is preferably 750 or less, more preferably 500 or less, further preferably 300 or less, further preferably 200 or less, and 100 or less. It is more preferable that there is.

On the other hand, when the alumina fibers are short fibers, the aspect ratio is preferably 5 or more. When the aspect ratio is 5 or more, when the composite sheet is manufactured, the fiber length is longer than the fiber diameter, so that the composite sheet excellent in mechanical strength and thermal conductivity can be manufactured even if the amount of the alumina fiber is small. This is because it is easy. It is more preferably 10 or more, still more preferably 20 or more, still more preferably 30 or more, still more preferably 40 or more. The “aspect ratio” is the ratio of the average fiber diameter of alumina fibers to the average fiber length, and is the value obtained by dividing the average fiber length (unit: μm) of alumina fibers by the average fiber diameter (unit: μm). Furthermore, "average fiber length" refers to the arithmetic mean value of the fiber lengths of 50 alumina fibers, and "fiber length" is the value of the alumina fibers measured based on 50 to 5000 times electron micrograph of the alumina fibers. The length in the length direction.

When the alumina fiber of the present invention has an aspect ratio of 1000 or less as described above, it is preferable that the CV value of the fiber length is 0.7 or less and the fiber length is uniform. When the fiber length is uniform in this way, when manufacturing a composite sheet, the alumina fibers are less likely to aggregate, and by being uniformly dispersed throughout the composite sheet, percolation can be secured and the composite sheet has excellent thermal conductivity. Is easy to manufacture. Since the smaller the CV value of the fiber length, the more uniform the fiber length, the CV value of the fiber length is preferably 0.6 or less, more preferably 0.5 or less. , 0.4 or less, more preferably 0.3 or less, still more preferably 0.2 or less, and ideally 0. The CV value of the fiber length is a value obtained by dividing the standard deviation of the fiber length by the average fiber length, that is, (standard deviation of fiber length/average fiber length). The "standard deviation" is a value obtained from the fiber length of 50 alumina fibers at the time of measuring the average fiber length.

Further, when the alumina fiber of the present invention is a short fiber, the rate of change in fiber length is preferably 30% or less. This is because such an alumina fiber has excellent mechanical strength and is not easily broken by pressure or shearing force, so that percolation can be secured even when a composite sheet is produced, and high thermal conductivity can be easily exhibited. .. That is, the fact that the rate of change in fiber length is small means that the average fiber length is unlikely to change even when pressure is applied, that is, the alumina fiber is less likely to be broken, as can be understood from the equation below. Is excellent in mechanical strength, and percolation can be secured even when a composite sheet is manufactured, and thus it is easy to manufacture a composite sheet exhibiting high thermal conductivity. Since the smaller the fiber length change rate is, the better the mechanical strength of the alumina fiber itself is, the fiber length change rate is preferably 20% or less, more preferably 15% or less. It is preferably 0%, ideally.

This fiber length change rate (Lr) is a value calculated from the following equation (1).
Lr=[(Lb−La)/Lb]×100 (1)
Lb means the average fiber length of the alumina fibers, and La means the average fiber length after applying a pressure of 10 MPa to the alumina fibers. The pressure applied to the alumina fiber is such that about 1 g of the alumina fiber is applied, and a pressure of 10 MPa is applied for 3 seconds at room temperature using a pressing machine.

The alumina fiber is in a sintered state so as to have excellent mechanical strength and thermal conductivity.

Further, although the alumina fiber is literally composed of aluminum oxide (Al ₂ O ₃ ), when it is combined with an organic resin to form a composite sheet, in order to enhance the affinity between the alumina fiber and the organic resin, The surface of the alumina fiber may be modified with a surface treatment agent such as a silane coupling agent.

The alumina fiber of the present invention can be produced, for example, by the method for producing an alumina fiber according to the present invention.

(Alumina fiber sheet of the present invention)
The alumina fiber sheet of the present invention is a fiber sheet made of the alumina fiber of the present invention. The alumina fiber sheet of the present invention has at least a bending strength of 5 MPa or more so as to have excellent strength, or has a bending deflection amount of 0.6 mm or more so as to have excellent flexibility, and preferably Has a bending strength of 5 MPa or more and a bending deflection amount of 0.6 mm or more.

Since the bending strength of the alumina fiber sheet of the present invention is 5 MPa or more, preferably 8 MPa or more, more preferably 10 MPa or more, the alumina fiber sheet of the present invention has excellent strength.
Further, the bending amount of bending in the alumina fiber sheet of the present invention is 0.6 mm or more, preferably 0.8 mm or more, and more preferably 1.0 mm or more, the alumina fiber sheet of the present invention is flexible. Is excellent.

The "bending strength" and the "bending flexure amount" in the present specification use a load displacement measuring device (for example, a load displacement measuring unit FSA-1KE-5N, manufactured by Imada Co., Ltd.) as a measuring device and follow the procedure below. Means the value determined by:
(1) The alumina fiber sheet is cut, and five 40 mm×7 mm test pieces are collected.
(2) Prepare a metal rod-shaped fulcrum having a rounded tip of 2±0.2 mm, and arrange the rod-shaped fulcrums so that the distance between the rod-shaped fulcrums is 16 mm.
(3) The test piece is arranged so as to straddle the rod-shaped fulcrums.
(4) Using a metal rod-shaped pressure wedge having a roundness of 5±0.1 mm at the center portion between the rod-shaped fulcrums of the test piece (center point of 8 mm from one rod-shaped fulcrum), speed 1 mm /Min. Then, the load when the center of the test piece is broken is measured up to 0.001N.
(5) Based on the above result, the bending strength (S, unit: MPa) is calculated from the following formula.
S=3PL/(2Wh ² )
Here, P is the load (unit: N) when the test piece is broken, L is the distance between rod-shaped fulcrums (=16 mm), W is the width of the test piece (=7 mm), and h is the thickness of the test piece (unit: : Mm), respectively.
(6) The bending strength is measured on five test pieces, and the arithmetic mean value thereof is defined as "bending strength".
(7) In the measurement of the bending strength, the bending amount (mm) of the pushing amount (=the distance between the initial position of the upper surface or the lower surface of the test piece and the broken position) by the rod-shaped pressure wedge when the test piece is bent And
(8) The bending deflection amount is measured for five test pieces, and the arithmetic mean value thereof is defined as "bending deflection amount".

The alumina fiber sheet of the present invention can be obtained, for example, as an intermediate product in the method for producing alumina fibers of short fibers according to the present invention.

(Alumina fiber-organic resin composite sheet of the present invention)
The alumina fiber-organic resin composite sheet of the present invention (hereinafter referred to as the composite sheet of the present invention) includes a composite sheet in which the alumina fiber of the present invention is dispersed in an organic resin. Further, the composite sheet of the present invention includes a composite sheet containing the alumina fiber sheet of the present invention in an organic resin. Furthermore, the composite sheet of the present invention includes a composite sheet containing the alumina fiber sheet of the present invention in an organic resin and having the alumina fiber of the present invention dispersed therein.

Since the organic resin constituting the composite sheet of the present invention varies depending on the application, it is not particularly limited. For example, when the composite sheet is used for a semiconductor device application or a thermal printer application, an epoxy resin or a polyimide resin is used. Examples thereof include fluorine resin, acrylic resin, polyester resin, and silicone polyester resin. When used for adhesives, styrene elastomers, polyamides, polyesters, polyurethanes, polyolefins, etc. can be exemplified. Furthermore, when used for solar cell applications, ethylene/vinyl acetate copolymer (EVA), polyimide, polyester resin and the like can be exemplified.

When the composite sheet of the present invention is a composite sheet in which alumina fibers are dispersed, the proportion of alumina fibers in the composite sheet is preferably 50% by volume or less. If the proportion of the alumina fibers exceeds 50% by volume, the alumina fibers tend to agglomerate, and variations in mechanical strength and thermal conductivity tend to occur. More preferably, it is 48% by volume or less, and further preferably Is 45% by volume or less, more preferably 40% by volume or less, further preferably 30% by volume or less, and further preferably 20% by volume or less. On the other hand, the alumina fiber content is preferably 3% by volume or more, more preferably 5% by volume or more, and further preferably 8% by volume or more so that the composite sheet has excellent mechanical strength. It is more preferably 10% by volume or more.

When the composite sheet of the present invention contains an alumina fiber sheet, the proportion of the alumina fiber sheet in the composite sheet is preferably 50% by volume or less. When the proportion of the alumina fiber sheet exceeds 50% by volume, it tends to be difficult to allow the organic resin to uniformly exist in the entire composite sheet. Therefore, it is more preferably 48% by volume or less, further preferably 45% by volume. Or less, more preferably 40% by volume or less, further preferably 30% by volume or less, further preferably 20% by volume or less. On the other hand, the alumina fiber sheet is preferably 1% by volume or more, more preferably 3% by volume or more, further preferably 5% by volume or more so that the composite sheet has excellent mechanical strength. preferable.

The composite sheet of the present invention contains an alumina fiber and/or an alumina fiber sheet and an organic resin, but may contain non-fibrous alumina in addition to the alumina fiber and/or the alumina fiber sheet. For example, it may contain alumina in the form of particles, plates, needles, and flakes. This is because such non-fibrous alumina exists so as to bridge the alumina fibers and further improves the thermal conductivity.

As described above, the alumina fiber and/or the alumina fiber sheet is contained in the above-mentioned volume amount so as to have excellent mechanical properties and thermal conductivity even when it contains non-fibrous alumina. Is preferred. When the non-fibrous alumina is contained, the total amount of the alumina fiber and/or the alumina fiber sheet and the non-fibrous alumina is a composite sheet so as not to impair the fluidity and moldability of the organic resin. The volume is preferably 80% or less.

The volume ratio (P) of the alumina fiber and/or the alumina fiber sheet in this composite sheet is a value calculated by the following equation (2).
P=(F/H)×100 (2)
In the formula, F means the volume of the alumina fiber and/or the alumina fiber sheet, and H means the volume of the composite sheet.

The composite sheet of the present invention has an alumina fiber and/or an alumina fiber sheet and is excellent in mechanical strength, and also has a thermal conductivity of 3 W/m·K or more at a thickness of 0.3 mm. It is preferably excellent in properties, more preferably 3.1 W/m·K or more, further preferably 3.2 W/m·K or more, and further preferably 3.3 W/m·K or more. Yes, and more preferably 3.4 W/m·K or more.

In addition, in the present invention, the thermal conductivity is the thermal conductivity at a thickness of 0.3 mm, but the thermal conductivity of the polymer material can be measured by a thermal conductivity measuring device described later. This is because it is 0.3 mm or more.

The thermal conductivity is a value measured as follows.
(1) A sample piece having a length of 10 mm, a width of 10 mm, and a thickness of 0.3 mm is collected from the composite sheet. When a sample piece having a length of 10 mm, a width of 10 mm, and a thickness of 0.3 mm cannot be collected, a sample piece having a length of 1 mm or more, a width of 1 mm or more, and a thickness of 0.3 mm is substituted. However, there must be a smooth portion in the vertical and horizontal directions of the sample piece.
(2) After measuring the thermal diffusivity, specific heat and density of this sample piece by the following method, the thermal conductivity of the sample piece (composite sheet) is determined by the following formula (A).

<Thermal diffusivity>
Using a thermal conductivity measuring device (registered trademark: ai-Phase Mobile, manufactured by Eye Phase Co., Ltd.), measurement is performed at room temperature by a temperature wave thermal analysis method. When the thickness of the composite sheet is less than 0.3 mm and a sample piece having a thickness of 0.3 mm cannot be sampled, a value converted into a thickness of 0.3 mm is measured by a thermal conductivity measuring device.

<Specific heat>
It is measured by comparison with a sapphire standard material using a differential scanning calorimeter (DSC).

<Density>
It is measured using the Archimedes method.

<Thermal conductivity>
Thermal conductivity = (thermal diffusivity) x (specific heat) x (density)...(A)

The composite sheet of the present invention is one in which alumina fibers are dispersed in an organic resin, or one containing an alumina fiber sheet in an organic resin, and as long as it is in a sheet form, its form is particularly limited. However, it can be a two-dimensional form such as a thin film having a thickness of 1 mm or less, or a plate having a thickness of more than 1 mm. In particular, when the composite sheet of the present invention contains an alumina fiber sheet composed of an alumina fiber having an average fiber diameter of 3 μm or less and a certain amount of fine alumina fibers, or an alumina fiber sheet having an average fiber diameter of 3 μm or less and an amount of a fine alumina fiber, the thickness is 1 mm. It can be a thin film such as:

Such a composite sheet of the present invention can be manufactured by a conventional method. For example, after adding short-fiber alumina fibers to a solution in which an organic resin is dissolved and, if necessary, non-fibrous alumina, to prepare an alumina fiber dispersion, and then supporting the alumina fiber dispersion. It can be applied to the body, dried and subsequently peeled from the support to produce a composite sheet having a two-dimensional aspect.

Further, to the melt obtained by melting the organic resin, alumina fibers of short fibers are added, and if necessary, alumina in a non-fiber state is also added to prepare an alumina fiber-dispersed melt, followed by molding, It is also possible to produce composite sheets with a dimensional aspect.

Furthermore, a solution in which an organic resin is dissolved, if necessary, a solution in which alumina fibers of short fibers and/or alumina in a non-fiber state are added is impregnated or applied to an alumina fiber sheet and dried, Composite sheets having a two-dimensional aspect can be produced. Alternatively, after applying a solution similar to the above to the support, an alumina fiber sheet is laminated on the application solution, and if necessary, the solution similar to the above is applied to the alumina fiber sheet and dried, Composite sheets having a two-dimensional aspect can be produced.

(Production method of the present invention)
In the present invention, the production method of the alumina fiber,
(I) a step of spinning a spinnable sol solution prepared by hydrolyzing a solution containing an aluminum alkoxide at a temperature of 5° C. or lower and then polycondensing
(Ii) a step of sintering the fiber spun in (i) above at a temperature of 1100° C. or lower,
including.

According to the production method of the present invention, for example, a spinnable sol solution prepared by hydrolyzing a solution containing an aluminum alkoxide at a temperature of 5° C. or lower and then polycondensing the same is spun from an alumina fiber. The following alumina fiber sheet is formed, and the spun fiber can be sintered at a temperature of 1100° C. or lower to produce a sheet-shaped alumina fiber. That is, it is possible to manufacture an alumina fiber which is composed of 95 vol% or more of α-alumina and has a crystallite size of 500 Å or less. In the case of producing alumina fibers of short fibers, the alumina fiber sheet can be obtained by subsequently pressing and crushing the alumina fiber sheet with a pressing machine.

More specifically, first, a spinnable sol solution is spun to form an alumina fiber sheet made of alumina fibers. Although the spinning method is not particularly limited, for example, an electrostatic spinning method, a gas is parallel to the spinning solution discharged from the liquid discharging section as disclosed in JP-A-2009-287138. A method in which the fiber is discharged into a spinning solution and a shearing force is applied linearly to the spinning solution to form a fiber. Among these, according to the electrostatic spinning method, it is possible to spin a continuous fiber having an average fiber diameter of 3 μm or less and being thin to some extent. Further, according to the electrospinning method, an alumina-based fiber sheet having a small average pore diameter and uniform pore diameters can be formed, and therefore it is suitable for producing short-fiber alumina fibers. That is, in the alumina-based fiber sheet having a small average pore diameter and uniform pore diameter, the distance between the intersections of the alumina-based fibers is short and the distance between the intersections is uniform, so the alumina fiber sheet after sintering is crushed. As a result, it is easy to manufacture short-length alumina fibers having a uniform CV value of 0.7 or less as described above.

Regarding this point, FIG. 1 which is a plan view schematically showing the arrangement state of the alumina fibers in the alumina fiber sheet formed by the electrospinning method and the alumina fiber sheet formed by a method other than the electrospinning method. Referring to FIG. 2, which is a plan view schematically showing the arrangement state of alumina fibers, according to the electrostatic spinning method, as shown in FIG. 1, alumina having a small average pore diameter and uniform pore diameters is used. Since the system fiber sheet can be formed, the distance between the intersections of the alumina fibers is short and the distances between the intersections are uniform. For example, when viewed with c5, which is the intersection of fibers, as a reference, the distance from the intersections of alumina fibers adjacent to c5, b5, c4, c6, and d4, is relatively short, and the distances are almost the same. ..

On the other hand, as shown in FIG. 2, the alumina-based fiber sheet formed by a method other than the electrostatic spinning method has a large variation in pore diameter. For example, when viewed with C5, which is the intersection of fibers, as a reference, the distances from B5, C4, C6, and D4, which are the intersections of alumina fibers adjacent to C5, vary greatly.

This preferred electrostatic spinning method is a method in which an electric field is applied to a spinning stock solution (spinning sol solution) to draw the spinning stock solution to form fibers. The electrospinning method will be briefly described with reference to FIG. 3, which is a schematic cross-sectional view of the electrospinning apparatus disclosed in JP-A-2005-194675.

The electrostatic spinning device shown in FIG. 3 is a spinning stock solution supply device 1 capable of supplying a spinning stock solution to a nozzle 2, a nozzle 2 capable of discharging the spinning stock solution supplied from the spinning stock solution supply device 1, a nozzle 2 and is drawn by an electric field. And a voltage applying device 4 capable of applying a voltage to the nozzle 2 to form an electric field between the nozzle 2 and the grounded collector 3, and the nozzle 2 The spinning container 6 containing the collector 3 is provided, a gas supply device 7 capable of supplying a gas having a predetermined relative humidity to the spinning container 6, and an exhaust device 8 capable of exhausting the gas in the spinning container 6.

In such an electrostatic spinning device, the spinning dope is supplied to the nozzle 2 by the spinning dope supply device 1. The supplied spinning solution is discharged from the nozzle 2 and subjected to a stretching action due to an electric field between the grounded collector 3 and the nozzle 2 applied by the voltage application device 4, to the collector 3 while being formed into fibers. Fly towards. Then, the flying alumina-based fibers are directly accumulated on the collector 3 to form an alumina-based fiber sheet.

As the spinning stock solution, a spinnable sol solution is used. This spinnable sol solution can be obtained, for example, by hydrolyzing a solution (a raw material solution) containing an aluminum alkoxide at a temperature of 5° C. or lower and then polycondensing it. The solvent of the raw material solution can be, for example, an organic solvent (for example, alcohol) and/or water.

The "spinnability" is determined by performing electrostatic spinning under the following conditions and by the following criteria.

(Judgment method)
A solution (solid content concentration: 10 to 50 mass%) for judging spinnability is discharged from a metal nozzle (inner diameter: 0.4 mm) arranged in a horizontal direction onto a grounded metal plate (discharge amount: 0.5 to 1.0 g/hr), and a voltage is applied to the nozzle (electric field strength: 1 to 3 kV/cm, polarity: positive or negative application) for 1 minute or more without causing solidification of the solution at the tip of the nozzle. The fibers are accumulated on the metal plate by continuously spinning.
There is a condition that a fiber having an average fiber diameter (arithmetic average value of 50 points) of 5 μm or less and an aspect ratio of 100 or more can be produced by observing and observing a scanning electron micrograph of the accumulated fibers. If so, the solution is judged to be “spinnable”. On the other hand, no matter how the above conditions (that is, concentration, extrusion amount, electric field strength, and/or polarity) are changed and combined, there are droplets, oil-like uniform fiber form, average fiber diameter Is more than 5 μm, or the aspect ratio is less than 100 (for example, in the form of particles) and the conditions for producing the fiber do not exist, the solution is determined to be “no spinnability”.

This spinnable sol solution preferably has a viscosity of 0.01 to 10 Pa·s, more preferably 0.05 to 5 Pa·s, and more preferably 0.1 to 3 Pa·s so that electrostatic spinning can be performed. More preferably, it is s. This is because if the viscosity exceeds 10 Pa·s, it is difficult to spin fine alumina fibers, and if it is less than 0.01 Pa·s, the fiber shape itself tends to be unobtainable. When the spinning of the spinnable sol solution is performed by using a nozzle, the spinnability of the spine having a viscosity of more than 10 Pa·s is set by setting the atmosphere at the tip of the nozzle to be the same solvent gas atmosphere as the solvent of the raw material solution. In some cases, even a soluble sol solution can be spun.

This spinnable sol solution may contain an organic component in addition to the above-mentioned alumina component. For example, it may contain a silane coupling agent, an organic low molecular compound such as a dye, an organic polymer compound such as polymethylmethacrylate, or the like.

The raw material solution is a solvent that stabilizes the aluminum alkoxide contained in the raw material solution [for example, an organic solvent (for example, alcohols such as ethanol, dimethylformamide) or water], and hydrolyzes the aluminum alkoxide contained in the raw material solution. Water, and a catalyst (for example, tetrabutylammonium hydroxide, etc.) that smoothly promotes the hydrolysis reaction.

The raw material solution is, for example, a chelating agent for stabilizing aluminum alkoxide, a silane coupling agent for stabilizing aluminum alkoxide, a compound capable of imparting various functions such as piezoelectricity, transparency, and adhesiveness. An organic compound (for example, polymethylmethacrylate) for improving, flexibility, and adjusting hardness (fragility), an inorganic component such as hydroxyapatite, or an additive such as a dye can be included. Note that these additives can be added before the hydrolysis, during the hydrolysis, or after the hydrolysis.

In addition, the raw material solution may include inorganic or organic fine particles. Examples of the inorganic fine particles include titanium oxide, manganese dioxide, copper oxide, silicon dioxide, activated carbon, and metal (for example, platinum), and examples of the organic fine particles include dyes and pigments. The average particle size of the fine particles is not particularly limited, but is preferably 0.001 to 1 μm, more preferably 0.002 to 0.1 μm.

The amount of water for hydrolyzing the aluminum alkoxide contained in the raw material solution varies depending on the aluminum alkoxide, and is not particularly limited, but for example, in the case of aluminum ethyl acetoacetate diisopropylate, it is Therefore, the amount of water is preferably 4 times or less mol of the alkoxide.

Further, the reaction temperature is 5° C. or lower so that polycondensation during hydrolysis is suppressed, formation of a three-dimensional structure is suppressed, and a one-dimensional structure is easily formed, preferably 3° C. It is carried out at not higher than 0°C, more preferably not higher than 0°C.

The polycondensation following hydrolysis is not particularly limited as long as it is higher than the temperature at the time of hydrolysis, but the polycondensation can be carried out efficiently at a temperature of 25° C. or higher.

As the spinning dope solution feeder 1, for example, a syringe pump, a tube pump, a dispenser, or the like can be used. Further, instead of the nozzle 2, a saw gear, a wire, a slit, or the like can be used. Furthermore, although the collector 3 in FIG. 3 has a drum shape, it may have a conveyor shape. Further, although the collector 3 is grounded in FIG. 3, the nozzle 2 may be grounded and a voltage may be applied to the collector 3, or both the nozzle 2 and the collector 3 may be applied. Although the voltage is applied, the voltage may be applied so as to have a potential difference.

Further, as the voltage application device 4, for example, a DC high voltage generator or a Van de Graaff generator can be used, and the spinnable sol solution is spun into fiber without causing dielectric breakdown of air. The applied voltage is appropriately adjusted so that it can be achieved. Further, the polarity of the applied voltage may be either positive or negative, but it is possible to suppress the spread of the alumina fibers, uniformly disperse the alumina fibers, and to manufacture an alumina fiber sheet with a uniform pore size. In addition, the polarity is appropriately selected according to the characteristics of the spinnable sol solution.

In the electrostatic spinning device of FIG. 3, a gas supply device 7 (for example, a propeller fan, a sirocco fan, an air compressor, a blower having a temperature/humidity adjusting function, etc.) and an exhaust device 8 (for example, a fan) are provided in the spinning container 6. Since they are connected, the atmosphere in the spinning container 6 can be made constant, so that an alumina-based fiber sheet having a uniform fiber diameter can be manufactured.

As described above, the alumina fibers constituting the alumina fiber sheet obtained by spinning the spinnable sol solution have not yet been converted to α-alumina and have low thermal conductivity. Therefore, the alumina fiber is sintered to be converted to α-alumina to obtain an alumina fiber sheet made of alumina fiber.

This sintering can be performed using, for example, an oven or a sintering furnace, and is performed at a temperature of 1100° C. or lower, preferably 1050° C. or lower, and more preferably 1000° C. or lower. As described above, even at a temperature as low as 1100° C. or less, α-alumina can be formed due to the one-dimensional structure due to hydrolysis, and the growth of α-alumina crystallites can be suppressed. With a child size of 500 Å or less, grain boundaries are small, and alumina fibers excellent in strength, flexibility and thermal conductivity can be manufactured. Further, the sintering time is 1 to 5 so that α-alumina is sufficiently converted to exhibit excellent thermal conductivity, and the crystallite size does not exceed 500 Å due to excessive crystal growth. It is preferably time, more preferably 1 to 3 hours.

When the alumina fibers are short fibers, the alumina fiber sheet is then pressed by a press and crushed to consist of αvola 95% or more and the αalumina crystallite size is 500Å or less. Alumina fibers of fibers can be produced.

When the alumina fiber sheet derived from the alumina fiber sheet formed by the electrostatic spinning method as described above is pressed by a press machine and pulverized, the aspect ratio is 1000 or less, the CV value of the fiber length is 0.7 or less, and It is suitable because it is easy to produce an alumina fiber having a fiber length change rate of 30% or less. That is, as described above, the alumina fiber sheet formed by the electrospinning method has a small average pore diameter, uniform pore diameters, a short distance between intersections of alumina fibers, and a uniform distance between intersections. Therefore, the alumina fiber sheet obtained by sintering this alumina fiber sheet is in the same state. Therefore, with respect to the alumina fiber sheet, when pressed by a press so as not to change the orientation of the alumina fiber, the intersection of the alumina fibers is strongly pressed, the alumina fiber formed by sintering has high rigidity, Combined with the fact that the fibers are not easily deformed, they are easily broken at the intersections of the alumina fibers, so that it is possible to manufacture alumina fibers having a uniform fiber length. In other words, the intersection of the alumina fibers overlaps with each other, and microscopically corresponds to a portion where the thickness of the alumina fiber sheet is increased. It acts preferentially. Therefore, it is possible to manufacture alumina fibers having a uniform fiber length.

Regarding this point, although it is a plan view schematically showing the arrangement state of the alumina fibers in the alumina fiber sheet formed by the electrostatic spinning method, the arrangement state of the alumina fibers in the alumina fiber sheet after sintering is schematically shown. FIG. 1 is a plan view schematically showing the arrangement state of alumina fibers in an alumina fiber sheet formed by a method other than the electrostatic spinning method. If it is explained based on FIG. 2, which can also be considered as a plan view schematically showing the arrangement state of the alumina fibers in the sheet, for example, the intersection points a1 to a3, b1 to b5, c1 to c6 of the fibers in FIG. In d1 to d6 and e1 to e5, the two alumina fibers are in a state of intersecting each other, and therefore, the thickness is about twice as large as that in a portion where they do not intersect. Therefore, when pressure is applied to the alumina fiber sheet of FIG. 1 by a press, pressure is preferentially applied to intersections a1 to a3, b1 to b5, c1 to c6, d1 to d6 and e1 to e5 of the fibers. In combination with the rigidity of the sintered alumina fiber, the alumina fiber is broken at intersections a1 to a3, b1 to b5, c1 to c6, d1 to d6 and e1 to e5 of the fibers. Therefore, it is easy to manufacture an alumina fiber having a uniform CV value of fiber length (0.7 or less).

On the other hand, as shown in FIG. 2, similarly for the alumina fiber sheet formed by a method other than the electrostatic spinning method, the intersections A1 to A3, B1 to B5, C1 to C7, D1 to D6 and E1 of the fibers are similarly formed. In E5, since the two alumina fibers are in a state of intersecting each other, the thickness is about twice as large as that in a portion where they do not intersect. Therefore, when pressure is applied to the alumina fiber sheet as shown in FIG. 2 by a press, pressure is applied preferentially to the intersections A1 to A3, B1 to B5, C1 to C7, D1 to D6 and E1 to E5 of the fibers. In addition, the rigidity of the sintered alumina fiber is also taken into consideration, and the alumina fiber breaks at intersections A1 to A3, B1 to B5, C1 to C7, D1 to D6, and E1 to E5. Therefore, it is difficult to manufacture alumina fibers having a uniform CV value of fiber length (0.7 or less).

In addition, when FIGS. 1 and 2 are plan views of the alumina fiber sheet, FIGS. 1 and 2 schematically show the arrangement state of the alumina fibers in the alumina fiber sheet, and the intersections of the alumina fibers are two. The state where the alumina fibers intersect is shown, but in reality, there are intersections where three alumina fibers intersect and there are intersections where four or more alumina fibers intersect, so the thickness varies depending on the intersection. .. Therefore, a stronger pressure acts on an intersection having a larger number of intersections of alumina fibers. Therefore, when a weak pressure is applied, the alumina fibers are crushed at the intersection having a larger number of intersections of alumina fibers to increase the pressure to act. Accordingly, the alumina fibers are crushed even at intersections where the number of intersections of alumina fibers is small. Therefore, the fiber length of the alumina fiber can be controlled to some extent by the applied pressure. Further, when the fiber is spun by the electrostatic spinning method, it is possible to spin alumina fibers having a uniform fiber diameter to some extent. Therefore, by controlling the electrospinning conditions and the crushing conditions (particularly the pressure), it is possible to produce alumina fibers having a desired fiber length or a desired aspect ratio.

Hereinafter, the present invention will be described in detail with reference to Examples, but these do not limit the scope of the present invention.

<<Example 1: Preparation of alumina continuous fiber sheet A>>
Aluminum ethyl acetoacetate diisopropylate, tetrabutylammonium hydroxide, water and 2-propanol were mixed at a temperature of 0° C. to a molar ratio of 1:0.005:1.5:20, and at a temperature of 0° C. After stirring for 24 hours for hydrolysis, the mixture was heated and stirred at a temperature of 80° C. for 24 hours to cause polycondensation. Then, after concentrating with an evaporator until the alumina concentration became 20 mass %, the viscosity was increased to 2000 to 3000 mPa·s to obtain a spinnable alumina sol solution.
Then, using the spinnable alumina sol solution, an electrospinning apparatus as shown in FIG. 3 was used to spin under the following electrospinning conditions, followed by sintering under the following sintering conditions. An alumina continuous fiber sheet A made of alumina continuous fibers having a fiber diameter of 0.6 μm was obtained.

(Spinning conditions)
・Discharge amount from nozzle: 1.0 g/hour ・Distance between nozzle tip and drum collector: 10 cm
・Temperature and humidity in spinning container: 25°C/30%RH
・Applied voltage to the nozzle: +10kV
(Sintering conditions in a sintering furnace)
・1000°C/2 hours (alumina continuous fiber sheet A)

<<Example 1-2: Preparation of fibrous alumina filler A>>
From the alumina continuous fiber sheet A, a plurality of pieces of alumina fiber sheet having a mass of about 1 g were collected. Subsequently, these alumina fiber sheet pieces were overlapped to have a thickness of 1.5 cm, and then crushed by pressing with a press machine at a pressure of 5.5 MPa for 30 seconds to obtain fibers having the physical properties shown in Table 1. Alumina filler A was prepared.

<<Example 1-3: Preparation of fibrous alumina filler A-dispersed epoxy film>>
The fibrous alumina filler A was mixed with an epoxy resin so that the ratio of the fibrous alumina filler A in the final fibrous alumina filler A-dispersed epoxy film was 10% by volume, and the mixture was stirred with a stirrer for 10 minutes (rotation). (Number: 2000 rpm), a flat glass plate was formed into a film with a bar coater, and dried at a temperature of 145° C. for 30 minutes to prepare a fibrous alumina filler A-dispersed epoxy film.

<<Comparative Example 1: Preparation of Alumina Continuous Fiber Sheet A'>>
The procedure of Example 1 was repeated except that sintering was performed under the following sintering conditions to obtain an alumina continuous fiber sheet A′ made of alumina continuous fibers having an average fiber diameter of 0.6 μm.
(Sintering conditions in a sintering furnace)
・1600℃/2 hours (Alumina continuous fiber sheet A')

<<Comparative Example 2: Preparation of alumina continuous fiber sheets B and B'>>
Aluminum ethyl acetoacetate diisopropylate, tetrabutylammonium hydroxide, water, and 2-propanol were mixed at a temperature of 10° C. at a molar ratio of 1:0.005:1.5:20, and at a temperature of 10° C. Similar to the alumina continuous fiber sheets A and A', except that the mixture was stirred for 24 hours to be hydrolyzed, polycondensation and concentration were carried out to prepare a spinnable alumina sol solution, which was then spun by an electrostatic spinning method and then baked. By binding, alumina continuous fiber sheets B and B′ made of alumina continuous fibers having an average fiber diameter of 0.6 μm were obtained.

<<Comparative Example 2-2: Preparation of Fibrous Alumina Filler B>>
From the alumina continuous fiber sheet B, a plurality of alumina fiber sheet pieces having a mass of about 1 g were collected. Subsequently, these alumina fiber sheet pieces were overlapped to have a thickness of 1.5 cm, and then crushed by pressing with a press machine at a pressure of 5.5 MPa for 30 seconds to obtain fibers having the physical properties shown in Table 1. Alumina filler B was prepared.

<<Comparative Example 2-3: Preparation of Fibrous Alumina Filler B-Dispersed Epoxy Film>>
The procedure of Example 1-3 was repeated except that the fibrous alumina filler B was used instead of the fibrous alumina filler A to prepare a fibrous alumina filler B-dispersed epoxy film.

<<Comparative Example 3: Preparation of alumina continuous fiber sheets C and C'>>
Aluminum ethyl acetoacetate diisopropylate, tetrabutylammonium hydroxide, water, and 2-propanol were mixed at a temperature of 25° C. at a molar ratio of 1:0.005:1.5:20, and at a temperature of 25° C. Similar to the alumina continuous fiber sheets A and A', except that the mixture was stirred for 24 hours to be hydrolyzed, polycondensation and concentration were carried out to prepare a spinnable alumina sol solution, which was then spun by an electrostatic spinning method and then baked. When tied, the alumina continuous fiber sheets C and C′ could not be produced because they were crushed due to deformation due to shrinkage during sintering.

<<Comparative Example 3-2: Preparation of Fibrous Alumina Filler C>>
When the alumina continuous fiber sheet C was prepared, a mass of about 1 g of fibrous alumina powder crushed by deformation due to shrinkage during sintering was collected. On the other hand, a stainless steel cylindrical cylinder having an outer diameter of 5 cm, an inner diameter of 3.4 cm, and a height of 6.5 cm, and a fluororesin cylindrical rod having an outer diameter of 3.4 cm and a height of 7.5 cm. Prepared.
Then, after filling the bottom of the hollow portion of the cylinder with fibrous alumina powder to a thickness of 1.5 cm, the rod was inserted into the hollow portion of the cylinder, and in that state, it was set in a press machine and The fibrous alumina powder was crushed by pressing the rod at a pressure of 0.5 MPa for 30 seconds to prepare a fibrous alumina filler C having the physical properties shown in Table 1.

<<Comparative Example 3-3: Preparation of Fibrous Alumina Filler C-Dispersed Epoxy Film>>
The procedure of Example 1-3 was repeated except that the fibrous alumina filler C was used instead of the fibrous alumina filler A to prepare a fibrous alumina filler C-dispersed epoxy film.

<<Test Example 1: Evaluation of alumina continuous fiber sheet>>
For each of the obtained alumina continuous fiber sheets, the α-alumina ratio, the crystallite size of α-alumina, the bending strength, and the bending deflection amount were measured. The results are shown in Table 2 together with the hydrolysis temperature and the sintering temperature.

From the results of Table 2, since the alumina fiber of the present invention has an α-alumina ratio of 95 vol% or more, it has excellent thermal conductivity and a crystallite size of 500 Å or less, and therefore mechanical strength and flexibility. It turned out to be excellent.
It was also found that by setting the hydrolysis temperature to 5° C. or lower and the sintering temperature to 1100° C. or lower, it is possible to produce alumina fibers having a proportion of α-alumina of 95 vol% or more and a crystallite size of 500 Å or less.

<<Test Example 2: Measurement of thermal conductivity>>
Epoxy film (reference example), fibrous alumina filler A-dispersed epoxy film (Example 1-3), fibrous alumina filler B-dispersed epoxy film (comparative example 2-3), fibrous alumina filler C-dispersed epoxy film (comparative example) The measurement of the thermal conductivity of 3-3) was carried out by taking a test piece having a length of 10 mm, a width of 10 mm and a thickness of 0.3 mm from each film, and then in the (alumina fiber-organic resin composite sheet of the invention) column The thermal conductivity in the thickness direction was measured according to the described method for measuring thermal conductivity. The results are shown in Table 3.

From the results of Table 3, the filler-dispersed organic resin composite sheet in which the fibrous alumina filler composed of α-alumina of 95 vol% or more and α-alumina crystallite size of 500Å or less is dispersed in the organic resin has a thickness of 0.3 mm. Was excellent in thermal conductivity of 3 W/m·K or more.

Since the alumina fiber, the alumina fiber sheet, or the alumina fiber-organic resin composite sheet of the present invention has excellent thermal conductivity, applications requiring thermal conductivity, for example, semiconductor device applications, thermal printer applications, adhesive applications It can be used for solar cell applications.

1... Spinning liquid supply device; 2... Nozzle; 3... Collector;
4... voltage application device; 5... spinning space; 6... spinning container;
7... Gas supply device; 8... Exhaust device.

Claims (1)

(I) a step of spinning a spinnable sol solution prepared by hydrolyzing a solution containing an aluminum alkoxide at a temperature of 5° C. or lower and then polycondensing
(Ii) A method for producing an alumina fiber, which includes a step of sintering the fiber spun in the step (i) at a temperature of 1100° C. or lower.

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