JP2023184068A - Method for producing low-dielectric-tangent silica powder - Google Patents

Abstract

To provide a method for producing a silica powder having reinforcing properties of silica powder and a dielectric tangent close to a low dielectric tangent inherent in quartz glass.SOLUTION: A method for producing a low-dielectric-tangent silica powder in which the dielectric tangent of the silica powder is 0.0005 or less at 10 GHz and 0.0010 or less at 40 GHz includes a heating step in which a silica powder having an average particle size of 0.1 to 30 μm is placed in a heating furnace and heated in a vacuum or in a gas having a dew point of 15°C or less under conditions in which the maximum heating temperature is 100 to 1,000°C and the amount of heating represented by a heating temperature (°C) of 100°C or more×a heating time (h) under the above condition is 450 (°C h) or more.SELECTED DRAWING: None

Description

The present invention relates to a method for producing silica powder that has excellent dielectric properties, especially dielectric loss tangent in a high frequency range.

BACKGROUND ART Currently, as information terminals such as smartphones become more sophisticated and communicate at higher speeds, the printed wiring boards used are becoming denser, thinner, and have lower dielectric and dielectric loss tangents. The insulating material for this printed wiring board is a laminate made by laminating prepreg obtained by impregnating glass cloth with thermosetting resin such as epoxy resin (hereinafter referred to as "matrix resin") and curing it under heat and pressure. is widely used. Signal transmission loss in the board is described by Edward A. Wolff formula: Transmission loss ∝√ε×tanδ, as shown in It is known that the contribution of the dielectric loss tangent is large. Therefore, glass cloth with improved dielectric properties such as D glass, NE glass, L glass, and quartz glass is combined with a low dielectric thermosetting resin (Patent Document 1). A common method is to add an inorganic powder that has a lower dielectric loss tangent than the resin. However, there is no description of a low dielectric loss tangent silica powder having a dielectric loss tangent of 0.0010 or less at 10 GHz and 0.0015 or less at 40 GHz.

Silica powder, which is one of the typical general-purpose inorganic powders, has a small coefficient of expansion as an inorganic powder added to resins, and is a material with excellent insulation and dielectric properties. If the dielectric properties of silica powder, especially the dielectric loss tangent, can be lowered to the level of original silica glass, significant growth can be expected in the future as encapsulating materials for high-speed communication semiconductors, high-speed communication substrates, and fillings for antenna substrates, etc. It is thought that it can be used in a wide range of applications as an agent. For this purpose, silica powder having a dielectric loss tangent of 0.0005 or less at 10 GHz was discovered (Patent Document 2). However, since a strained layer is likely to be formed on the surface of the silica powder by treatment at high temperatures, the strength of the cured product of the resin composition filled with the silica powder is likely to decrease. Therefore, in order to remove the strained layer on the surface of the silica powder, an etching process is required in which the silica powder is immersed in an etching solution. Even with the treated silica powder, it was not possible to obtain one with the desired dielectric loss tangent.

Japanese Patent Application Publication No. 2021-195689 Japanese Patent Application Publication No. 2021-187714

The present invention has been made in view of the above problems, and it is possible to produce silica powder that has the reinforcing properties of silica powder and a dielectric loss tangent close to the low dielectric loss tangent inherent to silica glass, without an etching process. The purpose is to provide a method for

The present inventors have made extensive studies to achieve the above object. Silica powder has a strong surface Si-OH group (silanol group) activity, and especially in high-temperature atmospheres, it absorbs moisture through hydrogen bonds and cleaves Si-O-Si bonds, which generates additional Si-OH groups. (SiO ₂ +H ₂ O⇔Si-OH).

A gas furnace or an electric furnace is generally used as a heat source for a heating furnace for removing adsorbed water from silica powder. Since gas furnaces use the combustion of city gas as a heat source, large amounts of carbon dioxide and water are produced as products. As a result, it was found that by using a heating furnace that generates moisture as a heating mechanism, the equilibrium of the above reaction can be tilted in the direction in which Si--OH groups are produced.

Furthermore, the present inventors have found that it is necessary to reduce the Si--OH groups in order to lower the dielectric loss tangent of the silica powder. By using a heating furnace with a heat generating mechanism that does not generate moisture, and by controlling the dew point and heating and cooling in an atmosphere with low moisture, the increase in Si-OH groups can be prevented, and furthermore, the condensation of Si-OH groups due to heating can be prevented. It has been found that by removing the condensation water generated in step 1, it is possible to reduce the Si--OH groups and, as a result, to lower the dielectric loss tangent of the silica powder.

Based on the above findings, silica powder with an average particle size of 0.1 to 30 μm is placed in a heating furnace, and the maximum heating temperature is 100 to 1,000 °C in a vacuum or a gas with a dew point of 15 °C or less under the above conditions. It was discovered that the above problem could be solved by including a heating step in which the heating amount expressed by heating temperature (°C) x heating time (h) of 100°C or more was 450 (°C / h) or more. However, the present invention has been completed.

Therefore, the present invention provides a method for producing a low dielectric loss tangent silica powder.
1. Silica powder with an average particle size of 0.1 to 30 μm is placed in a heating furnace, and the maximum heating temperature is 100 to 1,000 °C and 100 °C or higher under the above conditions in vacuum or in a gas with a dew point of 15 °C or less. The dielectric loss tangent of the silica powder is 0.0005 or less at 10 GHz, including a heating step in which the heating amount expressed as heating temperature (°C) x heating time (h) is 450 (°C / h) or more. , and a low dielectric loss tangent of 0.0010 or less at 40 GHz.
2. 2. The method for producing a low dielectric loss tangent silica powder according to 1, wherein dry gas having a dew point of 0° C. or less is introduced into the heating furnace at least one of the following steps: before heating, during temperature increase, during temperature maintenance, and during temperature decrease.
3. 2. The method for producing a low dielectric loss tangent silica powder according to 2, wherein dry gas having a dew point of 0° C. or lower is introduced into the heating furnace during cooling.
4. 4. The method for producing a low dielectric loss tangent silica powder according to any one of 1 to 3, wherein the gas is selected from air and inert gas and has a dew point of 15° C. or lower.
5. The low dielectric loss tangent silica powder according to any one of 1 to 4, wherein the heating furnace has a heat generation mechanism such that the amount of water generated per unit calorific value (1,000 kcal) is 0.12 L or less. How the body is manufactured.
6. 6. The method for producing a low dielectric loss tangent silica powder according to any one of 1 to 5, wherein the content of silicon-bonded hydroxyl groups (Si-OH) in the low dielectric loss tangent silica powder is 300 ppm or less.
7. 7. The method for producing low dielectric loss tangent silica powder according to any one of 1 to 6, wherein the maximum particle size of the low dielectric loss tangent silica powder is 100 μm or less.
8. 8. The method for producing a low dielectric loss tangent silica powder according to any one of 1 to 7, further comprising a step of treating the surface of the heat-treated low dielectric loss tangent silica powder with a coupling agent after the heating step.

According to the present invention, it is possible to produce silica powder that has the reinforcing properties of silica powder and a dielectric loss tangent close to the low dielectric loss tangent inherent to silica glass. The silica powder obtained by this method has the remarkable effect of suppressing the transmission loss of substrates used in high-speed communications such as 5G, which will increase in the future.

FIG. 3 is a diagram showing an example of a dry gas introduction device. It is a figure showing the calculation method of the amount of heating of the present invention. It is a figure explaining the calculation method of the dielectric loss tangent of silica powder.

The present invention will be explained in detail below.
[Raw material silica powder]
The silica powder that is the raw material for the low dielectric loss tangent silica powder has an average particle size of 0.1 to 30 μm. Silica powder is fused silica powder, which is made by crushing naturally occurring crystalline quartz and passing it through a high-temperature flame of around 2,000°C to make it spherical. Silica powder sintered and pulverized can be used, but any silica powder can be used regardless of the manufacturing method described above. Note that as the semiconductor encapsulant, it is preferable that the powder be in a spherical shape because it can be highly filled into the resin, but a powder in a crushed shape can also be used.

The raw material silica powder has an average particle size of 0.1 to 30 μm, preferably 0.5 to 20 μm. The maximum particle size is preferably 100 μm or less, more preferably 50 μm or less. If the average particle size is less than 0.1 μm, the specific surface area is large and the resin cannot be highly filled, and if it exceeds 30 μm, the ability to fill into narrow areas is poor and problems such as non-filling occur. In the present invention, the maximum particle size and the average particle size can be measured using a laser diffraction particle size distribution analyzer (for example, SALD-3100: manufactured by Shimadzu Corporation, etc.), and the average particle size can be measured using a laser light diffraction method. It can be determined as the mass average value D ₅₀ (that is, the particle diameter or median diameter when the cumulative mass is 50%) in the particle size distribution measurement.

Since the dielectric properties of the silica powder become more preferable by keeping the impurity concentration low, aluminum, magnesium, titanium and their oxides, and B in the inside and surface of the low dielectric loss tangent silica powder of the present invention The content of (boron), P (phosphorus), U (uranium), and Th (thorium) is preferably smaller. Further, from the viewpoint of preventing corrosion, the content of alkali metals and alkaline earth metals is preferably smaller. Furthermore, from the viewpoint of preventing malfunctions due to radiation, the content of U (uranium) and Th (thorium) is preferably smaller. In order to adjust these amounts of the low dielectric loss tangent silica powder, the raw material silica powder can be selected as appropriate.

[Heating process]
The heating step in the production method of the present invention is performed in a vacuum or in a gas with a dew point of 15°C or lower, with a maximum heating temperature of 100 to 1,000°C, and a heating temperature (°C) of 100°C or higher in the gas x heating time. This is a heating step in which heating is performed under conditions such that the heating amount represented by (h) is 450 (°C.h) or more.

(heating furnace)
The heating furnace used for heating can be one that can heat to 100 to 1,000°C and can create a vacuum or a dry gas atmosphere with a dew point of 15°C or less in the furnace. There is no particular limitation as long as it is a furnace, and examples of heating furnaces include gas furnaces, electric furnaces, muffle furnaces, laser heating, and the like.

Among these, it is preferable to use a heating furnace having a heat generating mechanism such that the amount of water generated per unit calorific value (1,000 kcal) is 0.12 L or less. As long as it has such a heat generating mechanism, it is not particularly limited, and examples thereof include an electric furnace, a muffle furnace, a laser heating, and the like, which include a heating furnace having a heat generating mechanism capable of the above. In particular, since electric furnaces do not involve combustion, the amount of water in the gas can be kept at 0.12 L or less, particularly less than 0.10 L.

It is preferable that the heating furnace has a device for feeding dry gas into the furnace. This device includes a heating furnace 1 containing silica powder 2, a silica powder 2 housed in a container such as alumina, a mechanism 4 for generating dry gas such as a compressor or an air dryer, and a device for filling or introducing dry gas. Examples include those having piping 3 that connects a mechanism for generating dry gas and the inside of the furnace, and a discharge mechanism 6 that exhausts air from the inside of the furnace. An example of introducing dry gas into a heating furnace is shown in FIG. However, the introduction method is not limited to this method.

(Heating atmosphere)
In the present invention, raw silica powder is heated in vacuum or in a gas having a dew point of 15° C. or lower. As the gas, inert gases such as air, nitrogen, and argon are preferred. When creating a vacuum inside the furnace, a vacuum heating and firing furnace such as a vacuum heating and firing furnace VASTA manufactured by Shimadzu Corporation can be used.

The method of filling the inside of the furnace with a gas with a dew point of 15°C or less is not particularly limited as long as the inside of the furnace is in a gas with a dew point of 15°C or less, but it is possible to fill the inside of the furnace with dry gas with a dew point of 15°C or less before heating. Alternatively, a method of introducing dry gas having a dew point of 15° C. or lower into the furnace may be mentioned. The introduction may be performed before heating, during temperature rise, during temperature maintenance, or during temperature fall, and a plurality of these may be selected and introduced. Among these, it is preferable to introduce a dry gas having a dew point of 15° C. or lower, preferably 0° C. or lower, into the furnace, and it is preferable to introduce the dry gas into the furnace during cooling. Examples of the drying gas include drying gases having a dew point of 15° C. or lower and selected from air, nitrogen, and inert gases such as argon. Among these, dry air is preferred in terms of production efficiency. Examples of devices that generate the dry air include a compressor, an air dryer, and the like. Note that the dew point in the present invention refers to an atmospheric pressure dew point. The dew point of the dry gas to be filled or introduced is preferably 15°C or lower (moisture content; 12.8 g/m ³ ), more preferably 0°C (water content: 4.85 g/m ³ ) or lower, and -20°C. The temperature is more preferably below (moisture content; 1.07 g/m ³ ), particularly preferably below -60°C (moisture content: 0.0193 g/m ³ ). In the heating step in gas, the lower the dew point, the more the equilibrium of the SiO ₂ +H ₂ O⇔Si-OH reaction tilts to the left, lowering the dielectric loss tangent of the quartz glass cloth.

The drying gas that is previously filled and introduced into the furnace before heating has a dew point of 15° C. or lower, preferably 0° C. or lower, and more preferably -20° C. or lower from the viewpoint of production efficiency and economic efficiency.
The dew point of the drying gas introduced into the heating furnace is preferably 15°C or less, and in order to further reduce the dielectric loss tangent of the quartz glass cloth, the dewpoint of the drying gas in the heating process from temperature rise to temperature drop is preferably 0°C or less, - The temperature is more preferably 20°C, and even more preferably -60°C or lower.

The amount of drying gas to be introduced is not particularly limited, but it is 0.5 to 20 times the volume of the furnace per hour to sufficiently lower the dew point inside the furnace and keep the temperature inside the furnace constant. preferable.

(Heating temperature)
The reaction of SiO ₂ +H ₂ O⇔Si-OH is activated at temperatures above 100°C, and as the temperature rises, the equilibrium shifts to the left and the Si-OH groups combine again to form a Si-O-Si bond. That is, the lower the dew point and the higher the heating temperature, the fewer Si--OH groups and the lower the dielectric loss tangent of the quartz glass cloth. However, the higher the heating temperature, the more distortion occurs on the surface of the silica powder, and the reinforcing effect of the silica powder decreases. Therefore, the maximum heating temperature of the silica powder in the present invention is 100 to 1,000°C, preferably 300 to 600°C, and more preferably 350 to 450°C. If the temperature is less than 100° C., sufficient energy cannot be applied to remove hydroxyl groups, and the dielectricity cannot be lowered. Moreover, if heating is performed above 1,000° C., the silica particles will fuse together and become large particles, which will cause problems in subsequent steps. A temperature of 1,000° C. or lower is sufficient to obtain the desired effect.

(Amount of heating)
The amount of heating expressed as heating temperature (°C) x heating time (h) of 100°C or higher in vacuum or in a gas with a dew point of 15°C or lower is 450 (°C·h) or higher. Note that the amount of heating does not include a range of less than 100°C unless it is in a vacuum or in a gas with a dew point of 15°C or less. FIG. 2 shows a method for calculating this amount of heating. The shaded area is the amount of heating, which is not affected by heating, holding, and cooling. If the amount of heat is less than 450 (℃・h), there is not enough time to tilt the equilibrium reaction of SiO ₂ + H ₂ O ⇔ Si-OH to the left, and the dielectric loss tangent of the quartz glass cloth does not decrease completely, making it unsuitable. . There is no particular limitation as long as the heating amount is 450 (℃・h) or more, but from the point of view of production efficiency, 450 to 50,000 (℃・h) is preferable, and 3,000 to 50,000 (℃・h) is preferable. More preferably, 4,000 to 45,000 (°C.h) is even more preferable.

Heating may be divided into several steps during temperature increase, temperature maintenance, and temperature reduction, and temperature may be maintained at a plurality of temperatures. Further, as long as the above heating amount can be satisfied, there is no need for holding time. There are no particular limitations on the temperature increase and temperature decrease rates, but from the viewpoint of productivity, it is preferably 10° C./h or more, and from the viewpoint of the strength of the silica powder, it is preferably less than 200° C./h.

In particular, in an atmosphere of 200 to 300°C, although the activation energy is exceeded, the equilibrium of SiO ₂ + H ₂ O ⇔ Si-OH tends to tilt to the right because it is a low temperature region, and the Si-O-Si bond is most likely to be cleaved. In particular, it is necessary to keep the dew point low. The drying gas may be introduced at any timing during temperature rise, temperature fall, or temperature maintenance. In order to keep the dew point in the furnace low, it is preferable to continue introducing dry gas into the furnace during the entire heating at 100 to 600° C. during heating, holding, and cooling. In particular, it is effective to introduce dry gas into the furnace during cooling from the maximum heating temperature to room temperature, particularly to 100° C., for improving the dielectric loss tangent.

[Coupling agent treatment process]
The manufacturing method of the present invention may further include a step of treating the heat-treated silica powder with a coupling agent after the heating step or the temperature-lowering step. This is a process in which the surface of the silica powder is treated with a coupling agent or the like. Through this step, part or all of the surface of the heat-treated silica powder is coated with the coupling agent, and when producing a resin composition etc. using the coupling agent-treated silica powder, the resin and the low dielectric loss tangent silica are coated. It is possible to strengthen the adhesion on the powder surface.

As the coupling agent, for example, known silane coupling agents can be used, and one type can be used alone or two or more types can be used in combination. Among them, alkoxysilanes are preferred, and vinyltrimethoxysilane, vinyltriethoxysilane, p-styryltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 2-(3,4 -Epoxycyclohexyl)ethyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxy Examples include silane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, and trifluoropropyltrimethoxysilane.

The silane treatment liquid is not particularly limited, but dry treatment using a Henschel or rocking mixer, or wet treatment using a 0.1 to 5% by mass dilute aqueous solution is preferred. The amount of the coupling agent is appropriately adjusted to 0.05 to 3% by mass based on the silica powder.

[Low dielectric loss tangent silica powder]
The dielectric loss tangent of the low dielectric loss tangent silica powder after the heating step, heating step, and temperature cooling step is 0.0005 or less at 10 GHz and 0.0010 or less at 40 GHz. At 10 GHz, it is preferably 0.0004 or less, more preferably 0.0003 or less. At 40 GHz, it is preferably 0.0009 or less, more preferably 0.0008 or less. The method for measuring the dielectric loss tangent is based on the resonance method, and specifically, it is based on the description of the examples described later.

The hydroxyl group (Si-OH) content of the low dielectric loss tangent silica powder is preferably 300 ppm or less, more preferably 280 ppm or less, and even more preferably 150 ppm or less. The method for measuring the hydroxyl group (Si-OH) content is based on the description in the Examples described below.

The content of aluminum, magnesium, titanium, and their oxides in the interior and surface of the low dielectric loss tangent silica powder is preferably 300 ppm or less, more preferably 150 ppm or less, and 100 ppm or less as aluminum, magnesium, and titanium metal mass. is even more preferable. By falling within this range, it is possible to obtain a silica powder that does not easily crystallize during the heat treatment process and has the desired low dielectric loss tangent.

The content of alkali metals and alkaline earth metals is preferably 10 ppm or less, more preferably 8 ppm or less, and even more preferably 5 ppm or less, each in terms of mass.
In the present invention, the alkali metal refers to lithium, sodium, potassium, rubidium, cesium, and francium, which are elements belonging to Group 1 in the periodic table, excluding hydrogen. In addition, alkaline earth metals refer to calcium, strontium, barium, and radium, excluding beryllium and magnesium, among elements belonging to Group 2 in the periodic table. Silica powder containing a large amount of alkali metals and alkaline earth metals has the problem of corroding the electrodes of high-speed communication boards and semiconductor devices, and from the viewpoint of corrosion prevention, silica powder containing a small amount of these metals is required.

The content of B (boron) inside and on the surface of the low dielectric loss tangent silica powder is preferably 2 ppm or less, more preferably 1 ppm or less. The content of P (phosphorus) is preferably 2 ppm or less, more preferably 1 ppm or less. The contents of U (uranium) and Th (thorium) are each preferably 0.1 ppb or less. In this way, by keeping the impurity concentration low, the dielectric properties and the like of the silica powder become more preferable.

The concentrations of aluminum, magnesium, titanium and their oxides, alkali metals and alkaline earth metals, as well as B (boron), P (phosphorus), U (uranium) and Th (thorium) can be determined by atomic absorption spectrometry, induction It can be measured by coupled plasma (ICP) emission spectroscopy or the like.

The shape and average particle size of the low dielectric loss tangent silica powder are similar to those of the raw material silica powder, and the average particle size is preferably 0.1 to 30 μm, more preferably 0.5 to 20 μm. The maximum particle size is preferably 100 μm or less, more preferably 50 μm or less. Note that the low dielectric loss tangent silica powder may be a blend of silica powders having different average particle diameters in order to improve properties such as fluidity and processability. When used as an underfill material or filler for high-speed substrates, the average particle size is preferably 0.1 to 5 μm and the maximum particle size is 20 μm or less, and the average particle size is 0.1 to 3 μm and the maximum particle size is 10 μm. The following are more preferable.

Silica powder with such excellent properties is suitable as a filler for semiconductor encapsulants, high-speed communication boards, antenna boards, and other substrates. Furthermore, by blending it with a resin, a low dielectric resin composition can be easily obtained. The low dielectric loss tangent silica powder is also useful as a filler for low dielectric organic substrates. Note that physical properties similar to those described above can be obtained also when the coupling agent treatment step is performed.

[Prepreg]
The above-described low dielectric loss tangent silica powder may be blended into thermosetting resins or thermoplastic resins as a filler for epoxy resins, silicone resins, polyimide resins, Teflon (registered trademark) resins, maleimide resins, polyphenylene ether resins, etc. Can be done. Further, prepreg formation using glass cloth or the like can be performed without any problem.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples and Comparative Examples, but the present invention is not limited to the Examples below.
The materials used in this example and comparative example are as follows.
<Silica powder>
(A) RS8225 (manufactured by Ryumorisha)
Average particle size: 15μm
Dielectric loss tangent (10GHz): 0.0008
(40GHz): 0.0013
Hydroxyl group content: 370ppm
(B) SO-E5 (manufactured by Admatec)
Average particle size: 1.5μm
Dielectric loss tangent (10GHz): 0.0011
(40GHz): 0.0015
Hydroxyl group content: 290ppm
(C) SO-E2 (manufactured by Admatec)
Average particle size: 0.5μm
Dielectric loss tangent (10GHz): 0.0023
(40GHz): 0.0031
Hydroxyl group content: 410ppm

[Example 1]
Put 5 kg of silica powder (A) (manufactured by Ryumori Co., Ltd. RS8225) with an average particle size of 15 μm, a dielectric loss tangent (10 GHz) of 0.0006, (40 GHz): 0.0013, and a hydroxyl group content of 370 ppm into an alumina container. When heat-treated at 400°C for 24 hours using an electric furnace B80 x 85 x 200-3Z12-10, the dew point was -20 using HITATHI's inverter package oil-free Bebicon POD-15VNP from temperature rise to temperature fall. Heat treatment was carried out by feeding dry air at a temperature of 5 times the volume of the electric furnace per hour. Further, the temperature increase rate was 100° C./h, the temperature decrease was 30° C./h, and thereafter the same temperature increase and temperature decrease rates were performed. The dielectric loss tangent and hydroxyl group content of the silica powder after heat treatment are also listed in the table.

[Example 2]
The silica powder (A) was mixed with a silica powder (B) (manufactured by Admatex, SO- E5), and the heat treatment was performed in the same manner as in Example 1, except that the heating time (h) was changed to 24 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (B) after heat treatment are also listed in the table.

[Example 3]
The silica powder (A) was mixed with a silica powder (C) (manufactured by Admatex, SO- E2), and the heat treatment was performed in the same manner as in Example 1, except that the heating time (h) was changed to 24 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (C) after heat treatment are also listed in the table.

[Example 4]
In the heat treatment of the silica powder (A) of Example 1, when heating at 400°C for 12 hours, the dry air with the dew point of Example 1 of -20°C was used with a dew point of CKD Co., Ltd.'s Super Heatless Dryer SHD3025. Heat treatment was carried out in the same manner as in Example 1 except that the temperature was changed to -70°C dry air. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 5]
In the heat treatment of the silica powder (A) of Example 1, the heat treatment was performed in air with a dew point of 20°C without introducing dry air when the temperature was raised or maintained, and when the temperature was lowered, dry air with a dew point of -20°C was supplied. The heat treatment was carried out in the same manner as in Example 1, except that the sample was sent into a furnace and heat treated. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 6]
In the heat treatment of the silica powder (C) in Example 3, dry air was not introduced when the temperature was raised or maintained, but the heat treatment was performed in air with a dew point of 20 °C, and dry air with a dew point of -20 °C was supplied when the temperature was lowered. The heat treatment was carried out in the same manner as in Example 3, except that the sample was sent into a furnace and heat treated. The dielectric loss tangent and hydroxyl group content of the silica powder (C) after heat treatment are also listed in the table.

[Example 7]
In the heat treatment of the silica powder (A) of Example 1, the heat treatment was carried out in the same manner as in Example 1, except that dry air was not introduced when the temperature was lowered, and the heat treatment was performed in air with a dew point of 20°C. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 8]
In the heat treatment of the silica powder (C) of Example 3, the heat treatment was carried out in the same manner as in Example 3, except that when the temperature was lowered, dry air was not introduced and the heat treatment was performed in air with a dew point of 20°C. The dielectric loss tangent and hydroxyl group content of the silica powder (C) after heat treatment are also listed in the table.

[Example 9]
The silica powder (A) was heated using a vacuum heating and calcining furnace VASTA manufactured by Shimadzu Corporation, which produces less than 0.1 L of water per unit calorific value (1000 kcal) instead of the electric furnace of Example 1. Heat treatment was performed at 400° C. for 12 hours in a vacuum state until the temperature decreased. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 10]
Heat treatment was performed in the same manner as in Example 1, except that the heating temperature and heating time of the silica powder (A) were changed from 400° C. and 12 hours to 150° C. and 48 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 11]
Heat treatment was performed in the same manner as in Example 1, except that the heating temperature and heating time of the silica powder (A) were changed from 400° C. and 12 hours to 700° C. and 7 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Example 12]
Heat treatment was performed in the same manner as in Example 3, except that the heating temperature and heating time of the silica powder (C) were changed from 400° C. and 24 hours to 900° C. and 5 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (C) after heat treatment are also listed in the table.

[Comparative example 1]
Silica powder (A) was heat-treated by heating it in air at a dew point of 20°C in the electric furnace at 400°C for 12 hours using an electric furnace B80 x 85 x 200-3Z12-10 manufactured by Nems. . The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Comparative example 2]
Heat treatment was performed in the same manner as in Example 1, except that the heating temperature and heating time of the silica powder (A) were changed from 400° C. and 12 hours to 80° C. and 32 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Comparative example 3]
Heat treatment was performed in the same manner as in Example 1, except that the heating temperature and heating time of the silica powder (A) were changed from 400° C. and 12 hours to 1,200° C. and 4 hours. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

[Comparative example 4]
Using the same equipment as in Example 1, dry air with a dew point of -20°C was fed in an amount of 5 times the volume of the electric furnace per hour, and silica powder (A) was placed in the electric furnace heated to 400°C. After a certain period of time, it was taken out without waiting for the temperature to cool down. The dielectric loss tangent and hydroxyl group content of the silica powder (A) after heat treatment are also listed in the table.

Further, the characteristic values in the present invention were measured by the following method.
<Measurement method of dielectric loss tangent>
The method for measuring dielectric loss tangent will be explained.
Silica powder was mixed with SLK-3000 (manufactured by Shin-Etsu Chemical Co., Ltd.), a low dielectric maleimide resin, and dicumyl peroxide (Percumyl D: NOF Corporation), a radical polymerization initiator, as a curing agent in the proportions shown in Table 1 below. A varnish was prepared by mixing, dispersing, and dissolving it in an anisole solvent containing (manufactured by ) Co., Ltd.).
Silica powder was added in the proportions shown in Table 1 below, and the volume% was 0%, 17.6%, 33.3%, and 48.1% based on the total amount of silica powder and resin (SLK-3000). An uncured maleimide resin composition was prepared by adding the anisole solvent, stretching it to a thickness of 200 mm using a bar coater, and placing it in a dryer at 80° C. for 30 minutes to remove the anisole solvent.

The prepared uncured maleimide resin composition was placed in a 60 mm x 60 mm x 100 μm mold, and after curing with a hand press at 180 ° C. for 10 minutes at 30 MPa, it was completely cured in a dryer at 180 ° C. for 1 hour. A cured resin sheet was prepared. The cured resin sheet was cut into a size of 50 mm x 50 mm, and the dielectric loss tangent at 10 GHz was measured using a dielectric constant measurement SPDR (Split post dielectric resonators) dielectric resonator frequency 10 GHz (manufactured by Keysight Technologies Co., Ltd.). .

As shown in Figure 3, the obtained dielectric loss tangent value is plotted by taking the volume % of the silica powder on the horizontal axis and the measured dielectric loss tangent on the vertical axis. I created a straight line with. This straight line was extrapolated, and the dielectric loss tangent of 100% by volume of silica powder was taken as the value of the dielectric loss tangent of the silica powder.

There are measuring devices that claim to be able to directly measure silica powder, but since the silica powder is filled in a measuring pot and the measurement is performed, it is difficult to remove air mixed in. In particular, silica powder with a large specific surface area is more difficult to obtain because it is greatly affected by air. Therefore, in order to eliminate the influence of the mixed air and obtain a value in a state close to the actual usage mode, in the present invention, the dielectric loss tangent of the silica powder was determined by the above-mentioned measurement method. The dielectric loss tangents at 10 GHz and 40 GHz in the Examples and Comparative Examples were calculated using similar calculations.

<Measurement method of hydroxyl group (Si-OH) content>
A sample was prepared by filling an aluminum pan with a thickness of 1.5 mm with silica powder up to the cutting edge. DRS-8000A) was used to measure the transmittance T at a peak around 3680 cm ^-1 caused by hydroxyl groups by a diffuse reflection method. Based on the obtained transmittance value, the Lambert-Beer law shown below was applied to determine the absorbance A.
・Absorbance A=-Log10T
Transmittance around T=3680 cm ⁻¹ Next, from the absorbance determined by the above formula, the molar concentration C (mol/L) of hydroxyl groups was determined by the following formula.
・C=A/εL
ε: molar absorption coefficient (molar absorption coefficient of hydroxyl group ε=77.5dm ³ /mol・cm)
C: Molar concentration (mol/L)
L: Sample thickness (optical path length) (1.5 mm)
The molar concentration C was determined from the obtained absorbance A using the above formula.
Using the obtained molar concentration C, the content (ppm) of hydroxyl groups in the silica powder was determined by the following formula.
・Hydroxyl group content (ppm) = {(C×M)/(d×1000)}×106
Specific gravity d of silica powder = 2.2g/cm ³
Molecular weight of hydroxyl group M (Si-OH) = 45g/mol

From Tables 2 and 3, the maximum temperature inside the furnace is 100 to 1,000°C, the inside of the furnace is in a vacuum atmosphere, or the dew point of the drying gas introduced into the furnace is low, the higher the heating temperature, and the drying when the temperature drops. By introducing air, silica powder with a lower dielectric loss tangent can be obtained.
In the heating at 80° C. in Comparative Example 2, the activation energy was insufficient, so the dielectric loss tangent did not change even after the heat treatment.

According to the present invention, silica glass having a dielectric loss tangent close to the low dielectric loss tangent of silica glass can be produced by controlling the dew point of the heating process, that is, by controlling the dew point during heating and cooling, without an etching process. A method for producing powder can be provided.

1 Heating furnace 2 Silica powder (in container)
3 Piping 4 Mechanism for generating dry gas 5 Dry gas 6 Discharge mechanism

Claims (8)

Silica powder with an average particle size of 0.1 to 30 μm is placed in a heating furnace, and the maximum heating temperature is 100 to 1,000 °C and 100 °C or higher under the above conditions in vacuum or in a gas with a dew point of 15 °C or less. The dielectric loss tangent of the silica powder is 0.0005 or less at 10 GHz, including a heating step in which the heating amount expressed as heating temperature (°C) x heating time (h) is 450 (°C / h) or more. , and a low dielectric loss tangent of 0.0010 or less at 40 GHz. The production of the low dielectric loss tangent silica powder according to claim 1, wherein dry gas having a dew point of 0° C. or less is introduced into the heating furnace at least one of the following steps: before heating, during heating, during temperature holding, and during temperature cooling. Method. 3. The method for producing low dielectric loss tangent silica powder according to claim 2, wherein a dry gas having a dew point of 0° C. or less is introduced into the heating furnace during cooling. 2. The method for producing a low dielectric loss tangent silica powder according to claim 1, wherein the gas is selected from air and an inert gas and has a dew point of 15° C. or lower. The method for producing low dielectric loss tangent silica powder according to claim 1, wherein the heating furnace is a heating furnace having a heat generation mechanism such that the amount of water generated per unit calorific value (1,000 kcal) is 0.12 L or less. . The method for producing a low dielectric loss tangent silica powder according to claim 1, wherein the content of silicon-bonded hydroxyl groups (Si-OH) in the low dielectric loss tangent silica powder is 300 ppm or less. The method for producing low dielectric loss tangent silica powder according to claim 1, wherein the maximum particle size of the low dielectric loss tangent silica powder is 100 μm or less. The method for producing a low dielectric loss tangent silica powder according to any one of claims 1 to 7, further comprising a step of treating the surface of the heat-treated low dielectric loss tangent silica powder with a coupling agent after the heating step. .

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