JP5854379B2 - Nanofiber - Google Patents

Description

  The present invention relates to nanofibers made of silicon nitride.

Silicon carbide nanofibers and silicon nitride nanofibers have been proposed as nanofibers having heat resistance (see, for example, Patent Document 1 and Patent Document 2).
These silicon carbide nanofibers and silicon nitride nanofibers can be produced by a CVD method.

  However, the silicon carbide nanofibers and silicon nitride nanofibers produced by the CVD method have been formed into thin and sharp needles, and there has been concern about the effects on the living body, so the commercialization has been abandoned.

JP 2004-360115 A ([0003]) JP 10-310500 A

In recent years, there is an increasing need for materials having heat resistance against high temperatures in energy saving technology, and expectations for nanofibers having heat resistance are increasing.
Therefore, there is a demand for a technique that does not affect the living body and a technique that prevents needle-shaped nanofibers from being present in an isolated state.

  In order to solve the above-described problems, the present invention provides a nanofiber having a heat resistance and a structure that does not affect a living body.

The nanofiber of the present invention is made of silicon nitride, has a diameter (fiber diameter) of 1 nm to 100 nm, is branched so that the nanofiber is branched into two, and is further separated at the point where the branched nanofiber is branched. It has a network structure by adhering to the nanofiber .

According to the above-described configuration of the nanofiber of the present invention, since the nanofiber is made of silicon nitride, the melting point (sublimation) of silicon nitride is as high as 1900 ° C., so that it has sufficient heat resistance.
In addition, the nanofibers are branched so as to be branched into two, and the branched nanofibers are attached to other nanofibers at the branch destination, so that the nanofibers have a network structure. It becomes possible not to affect.

According to the above-described present invention, it is possible to realize a nanofiber having a structure that has heat resistance and does not affect a living body.
Therefore, a safe heat-resistant material can be constituted using the nanofiber of the present invention.

It is a SEM photograph of the product of each sample which changed the kind of AC carbon black. It is the frequency distribution of the pore diameter of the carbon black used and each sample reaction product. It is a SEM photograph of the product of each sample which changed the mixture ratio of AC PMSQ. It is the TEM photograph of the product of each sample which changed the mixing ratio of AC PMSQ. It is a measurement result of the powder X-ray diffraction of each sample which changed the mixing ratio of PMSQ. It is the TEM photograph which expanded one nanofiber of each sample which changed the mixing ratio of AC PMSQ. AD is a TEM photograph of a nanofiber having a network structure of a sample with an addition amount of 50%. It is a TEM photograph of the nanofiber of the network structure of the sample of AC addition amount 50%. It is a TEM photograph of the nanofiber of the network structure of a sample with an addition amount of 50%. It is a SEM photograph of the product of the sample of A and B addition amount 10%. It is a SEM photograph of the product of the sample of A and B addition amount 20%. It is a SEM photograph of the product of the sample of A and B addition amount 30%. It is a SEM photograph of the product of the sample of A and B addition amount 40%. It is a SEM photograph of the product of the sample of A and B addition amount 50%. It is a SEM photograph of the product of the sample of A and B addition amount 60%. A and B are an SEM photograph and a TEM photograph of the upper product of the two-stage reaction cell. It is a figure which shows the relationship between the temperature of the result of mass spectrometry, and a weight change. It is a photograph of the conventional silicon carbide nanofiber.

Hereinafter, the best mode for carrying out the invention (hereinafter referred to as an embodiment) will be described.
The description will be given in the following order.
1. 1. Outline of the present invention Experiment

<1. Summary of the present invention>
First, the outline of the present invention will be described.
Silicon carbide nanofibers and silicon nitride nanofibers have been proposed as a kind of heat resistant material.
However, as described above, the conventionally proposed silicon carbide nanofibers and silicon nitride nanofibers are thin and pointed needles, and there is a concern about the influence on the living body.

A photograph of a conventional silicon carbide nanofiber is shown in FIG.
As shown in FIG. 18, since fine needle-like nanofibers are formed, it is considered to affect the living body.

Therefore, as a result of intensive studies aimed at realizing a nanofiber having a heat resistance and a structure that does not affect the living body, the present invention has been achieved.
The present invention provides a nanofiber having a network structure made of silicon nitride. The diameter (fiber diameter) of the nanofiber of the present invention is in the range of 1 nm to 100 nm.
The network structure of the nanofiber of the present invention is branched so that the nanofiber branches into two, and further, the two nanofibers with another nanofiber are attached at the branched end, It is a net-like structure.

According to the above-described configuration of the nanofiber of the present invention, since the nanofiber is made of silicon nitride, the melting point (sublimation) of silicon nitride is as high as 1900 ° C., so that it has sufficient heat resistance.
In addition, since the nanofiber has a network structure, it is possible to prevent the living body from being affected.
Therefore, a safe heat-resistant material can be constituted using the nanofiber of the present invention.

And since the nanofiber of the present invention has a network structure, for example, if the size of the opening of the network can be controlled, a heat-resistant filter that separates a substance that passes through the opening and a substance that does not pass through the opening is configured. It becomes possible to do.
The diameter (fiber diameter) of the nanofiber of the present invention was in the range of 1 nm to 100 nm as a result of observing 10 visual fields in the sample, extracting 90 to 150 fibers, and measuring the diameter.

The nanofiber of the present invention can be produced, for example, as follows.
First, carbon black and a compound containing silicon (hereinafter referred to as “silicon compound”) are mixed, and carbon black is impregnated with the silicon compound.
As the carbon black, those having many pores having a radius (pore diameter) of 30 nm or more are preferable, as will be described in detail later.
As the silicon-based compound, an organic compound containing silicon is preferably used. For example, polymethylsilsesquioxane (PMSQ) can be used.
When mixing carbon black and a silicon compound, a solvent that dissolves or disperses the silicon compound in a solvent is used as necessary. Then, after mixing with carbon black and impregnating carbon black with a silicon-based compound, the solvent is removed by heating.

Then, by heating the carbon black impregnated with the silicon compound in a nitrogen atmosphere, nitrogen and silicon can be reacted to form nanofibers made of silicon nitride.
Although the temperature of heat processing is based also on the silicon type compound to be used, when using the polymethylsilsesquioxane (PMSQ) mentioned above, it shall be 1000-1300 degreeC, for example.

  The mixing amount of carbon black and silicon compound depends on the silicon compound to be used. For example, when polymethylsilsesquioxane (PMSQ) is used, the amount (mass ratio) of PMSQ with respect to the whole is used. 50% to 60% is desirable.

When nanofibers are manufactured as described above, nanofibers made of silicon nitride are generated and carbon black remains.
The remaining carbon black can be decomposed and removed by reacting in an oxidizing atmosphere such as air or a reducing atmosphere such as hydrogen.

<2. Experiment>
Various production conditions were actually changed, and production conditions for forming the nanofiber of the present invention were examined.

(Experiment 1: Effect of carbon black properties on the product)
The product was examined by changing the properties of carbon black. Specifically, the reaction was performed as follows.
First, 1 g of polymethylsilsesquioxane (PMSQ) was dissolved in 100 ml of ethanol.
Next, carbon black was added so that the mixing ratio of polymethylsilsesquioxane (PMSQ) and carbon black (CB) would be PMSQ50% -CB50% (mass ratio), and stirred by a magnetic stirrer. did. In this way, carbon black was impregnated with PMSQ.
Thereafter, the solvent was removed by a rotary evaporator on a hot water bath set at 80 ° C., and the obtained solid was dried in an oven heated to 80 ° C. overnight.
Subsequently, heat treatment was performed at 1200 ° C. for 2 hours in a nitrogen atmosphere using an infrared high-temperature heating furnace (MR39-H / D, manufactured by ULVAC). The temperature rising rate was 10 ° C./min.
In this way, the reaction was carried out and the product was examined.

As carbon black, three types of carbon black (all made by Tokai Carbon) with different characteristics were used.
Hereinafter, the three types of carbon black used are referred to as CB1, CB2, and CB3. Table 1 summarizes the characteristics of the carbon blacks CB1, CB2, and CB3.

In Table 1, the DBP oil absorption is an oil absorption measured using DBP (dibutyl phthalate). Mesopores were 2 nm to 50 nm in diameter, and micropores were less than 2 nm in diameter.
The chemical structure of DBP (dibutyl phthalate) is as follows.

CB1 is standard carbon black.
CB2 has the same amount of DBP oil absorption as CB1, but its BET specific surface area is smaller than CB1.
CB3 has the same BET specific surface area as CB1, but the DBP oil absorption is smaller than CB1.
When the amount of micropores is large, the BET specific surface area becomes large. CB2 has a small BET specific surface area, and its mesopore volume and micropore volume are smaller than those of CB1 and CB3.
The DBP oil absorption has a positive correlation with the structure of carbon black.
That is, CB2 has the same structure as CB1, but has fewer pores. CB3 has a smaller structure than CB1.

The sample reacted in each nitrogen atmosphere was observed with SEM (scanning electron microscope). The obtained photograph is shown to FIG. 1A-FIG. 1C.
1A shows a case where CB1 is used, FIG. 1B shows a case where CB2 is used, and FIG. 1C shows a case where CB3 is used.

When CB1 is used, network-like nanofibers are formed as shown in FIG. 1A.
When CB2 is used, nanofibers are formed inside the aggregate as shown in FIG. 1B.
When CB3 is used, as shown in FIG. 1C, hexagonal columnar crystals of silicon nitride are formed between massive carbon blacks.

In addition, each carbon black simple substance (CB1, CB2, CB3), PMSQ and carbon black were mixed 50% each and then reacted in a nitrogen atmosphere (50% -CB1, 50% -CB2, 50%- For CB3), the frequency distribution of the pore diameter was measured.
The measurement results are shown in FIG. The horizontal axis of FIG. 2 shows the pore size (radius) r _p, the vertical axis represents the frequency distribution of each pore each diameter in volume distribution.

Here, in FIG. 2, attention is particularly paid to pores of 30 nm or more.
From FIG. 2, CB2 has pores of 30 nm or more smaller than CB1. CB3 has significantly fewer pores of 30 nm or more than CB1.
The sample mixed with PMSQ and reacted in a nitrogen atmosphere shows characteristics similar to those of the raw material carbon black for CB1 and CB2. However, CB3 has a mesomorphic property compared to the original carbon black CB3. The pores are greatly reduced.

From the above results, the product varies depending on the structure of carbon black and the amount of pores, and in particular, the distribution of pores having a radius (pore diameter) of 30 nm or more is considered to dominate the shape of the product.
When the ratio of pores having a radius (pore diameter) of 30 nm or more is large, mesh-like nanofibers are formed.
Further, as the proportion of pores having a radius (pore diameter) of 30 nm or more is reduced, nanofibers are formed inside, and when almost no, a silicon nitride crystal is formed on the surface of carbon black.

  In consideration of the above results, it is considered that even a carbon-based material other than carbon black may generate silicon nitride nanofibers if the amount of pores having a radius (pore diameter) of 30 nm or more is sufficient. .

(Experiment 2: Effect of mixing ratio on product 1)
Next, the carbon black to be used is CB1, and the PMSQ mixing ratio (ratio of PMSQ to the whole) is changed to 50%, 67%, and 80%, respectively, and the reaction is performed at 1200 ° C. in a nitrogen atmosphere. I checked things.

The scanning electron microscope (SEM) photograph of each product is shown to FIG. 3A-FIG. 3C.
3A shows a case where PMSQ is 50%, FIG. 3B shows a case where PMSQ is 67%, and FIG. 3C shows a case where PMSQ is 80%.
Moreover, the photograph by the transmission electron microscope (TEM) of each product is shown to FIG. 4A-FIG. 4C.
4A shows a case where PMSQ is 50%, FIG. 4B shows a case where PMSQ is 67%, and FIG. 4C shows a case where PMSQ is 80%.

When the amount of PMSQ added is 50%, a nanofiber having a network structure is formed. And the form of a nanofiber changes from a network structure to a linear structure with the increase in PMSQ addition amount. When 80% is added, in addition to nanofibers, those coated with carbon black are also formed.
As can be seen by comparing FIG. 4A and FIG. 4B, the nanofibers having a network structure and the nanofibers having a linear structure have different diameters, and the network nanofibers are more than the nanofibers having a linear structure. It is getting thinner.
Therefore, using TEM photographs, 10 visual fields were observed in each sample having PMSQ of 50% and 67%, 90 to 150 fibers were extracted, and the diameter was measured.
As a result of the measurement, 50% of the samples had a range of 1 nm to 100 nm and averaged 23 nm. In addition, 67% of the samples had a range of 10 nm to 300 nm and averaged 62 nm.

Further, powder X-ray diffraction measurement (according to JIS 1640) of each added amount of the sample was performed. The measurement results are shown in FIG. FIG. 5 also shows the measurement results of Si ₃ N ₄ crystals as a comparative control (using standard data).
From FIG. 5, a peak consistent with the crystal of Si ₃ N ₄ can be seen at any addition amount. From this, it is considered that silicon nitride is produced from nitrogen in the atmosphere and silicon of PMSQ.
Moreover, it turns out that crystallinity changes with the addition amount of PMSQ. When the addition amount is 67%, the crystallinity is relatively high, but when the addition amount is 50% and the addition amount is 80%, the crystallinity is low.
When the crystallite diameter was calculated using the peak at 2θ = 39 °, no significant difference was observed at around 700 mm for any sample.

Furthermore, the photograph by the transmission electron microscope (TEM) which expanded one nanofiber of the sample of each addition amount is shown to FIG. 6A-FIG. 6C.
6A shows a case where PMSQ is 50%, FIG. 6B shows a case where PMSQ is 67%, and FIG. 6C shows a case where PMSQ is 80%.
In the case of 67% in FIG. 6B, interference fringes indicating crystalline were observed. On the other hand, the case of 50% in FIG. 6A and the case of 80% in FIG. 6C were amorphous.

Furthermore, the photograph by the transmission electron microscope (TEM) of the nanofiber of the network structure of the sample of addition amount 50% is shown to FIG. 7A-7D, FIG. 8A-8C, and FIG.
FIG. 7B shows an enlarged view of the part enclosed by the rectangle in FIG. 7A, and FIG. 7D shows an enlarged view of the part enclosed by the rectangle in FIG. 7C.
8B shows an enlarged view of a portion surrounded by a rectangle in FIG. 8A, and FIG. 8C shows an enlarged view of a portion surrounded by the rectangle in FIG. 8B.

From the enlarged views of these TEM photographs, it is considered that a plurality of nanofibers do not overlap at the branching point of the mesh-like nanofibers, but branch from one nanofiber into two.
For example, in FIG. 7D, two nanofibers branched upward from one nanofiber partially overlap in the vicinity of the branch point. That is, it can be seen that the two separate nanofibers are not in a state of overlapping each other.

(Experiment 3: Effect of mixing ratio on product 2)
Next, in order to investigate the range of the mixing ratio of PMSQ and carbon black in which a nanofiber having a network structure was produced, the reaction was performed while changing the mixing ratio, and the product was examined.
The mixture ratio of PMSQ to CB1 (ratio of PMSQ with respect to the whole) was changed to 10%, 20%, 30%, 40%, 50%, 60%, respectively, and the reaction was performed at 1200 ° C. in a nitrogen atmosphere, The product was examined.

A photograph of each product by a scanning electron microscope (SEM) is shown in FIGS.
10A and 10B show the case where the PMSQ is 10%, FIGS. 11A and 11B show the case where the PMSQ is 20%, FIGS. 12A and 12B show the case where the PMSQ is 30%, and FIGS. Indicates a case where PMSQ is 40%, FIGS. 14A and 14B show a case where PMSQ is 50%, and FIGS. 15A and 15B show a case where PMSQ is 60%.

From FIG. 10A and FIG. 10B, in the case of PMSQ 10%, there is almost no formation of nanofibers.
From FIG. 11A and FIG. 11B, in the case of PMSQ 20%, a small amount of nanofibers are generated. In addition, other hexagonal column structures can be seen.
From FIG. 12A and FIG. 12B, in the case of PMSQ 30%, nanofibers are locally generated.
From FIG. 13A and FIG. 13B, when PMSQ is 40%, nanofibers are locally generated. There are more nanofibers than 30%.
From FIG. 14A and FIG. 14B, in the case of PMSQ 50%, nanofibers are generated as a whole.
From FIG. 15A and FIG. 15B, in the case of PMSQ 60%, nanofibers are generated as a whole. Some nanofibers can be seen that are partially transitioning linearly.

When these results are put together, when PMSQ is 10 to 40%, nanofibers are not formed, and even if they do not grow to take a network structure.
Moreover, when PMSQ is 50 to 60%, formation of a network-like nanofiber covering the carbon black surface is observed.
When the PMSQ is 67% or more, it is changed to a rigid nanofiber as in the case of 80% described above.
Therefore, the range of the amount of PMSQ added that the nanofibers having a network structure are generated is a very narrow range of 50 to 60%.

(Experiment 4: Investigation of reaction mechanism)
The nanofibers of the present invention are made of silicon nitride, and are produced by the reaction of PMSQ silicon and the nitrogen in the atmosphere. Therefore, the reaction for producing the nanofibers is considered to be a gas phase reaction.
Therefore, an experiment was conducted to investigate the reaction mechanism of nanofiber formation.

The reaction cell was divided into two stages, and the lower stage accommodated carbon black CB1 (PMSQ addition amount 80%) carrying PMSQ in a quartz cell.
This quartz cell was covered with a quartz mesh, and a quartz tube was placed on the quartz mesh to obtain an upper reaction cell.
And unsupported carbon black CB1 was accommodated in the upper quartz tube.
The two-stage reaction cell thus produced was heat-treated in a nitrogen atmosphere.
As a result, nanofibers were produced in the upper reaction cell.

16A and 16B show photographs taken by a scanning electron microscope (SEM) and a transmission electron microscope (TEM) of the product in the upper reaction cell, respectively.
From FIG. 16A and FIG. 16B, it turns out that the nanofiber is producing | generating other than the lump of carbon black.
That is, it can be seen that silicon nitride nanofibers were produced from the free silicon and nitrogen in the atmosphere even in the upper stage where the silicon component liberated from PMSQ passed through the quartz mesh and only carbon black was accommodated.
From this result, it can be seen that the movement of the gas phase component is involved in the formation of nanofibers.

Similarly, mass spectrometry was performed on a sample in which the amount of PMSQ added was 80%.
FIG. 17 shows the relationship between temperature and weight change (%) as a result of mass spectrometry.

In FIG. 17, it is considered that the mass reduction corresponding to the molecular weight 44 is a mass reduction due to SiO (silicon oxide), and the mass reduction corresponding to the molecular weight 16 is a mass reduction due to CH ₄ .
As a result of mass spectrometry, generation of SiO (300 to 1000 ° C.) and CH ₄ (620 to 940 ° C.) was observed from PMSQ.
SiO which is a raw material of SiO ₂ is generated from PMSQ between 300-1000 ° C. On the other hand, CH ₄ is produced at 620 to 940 ° C.
Moreover, it was found from the above-described experiment using a spatially isolated two-stage cell that the transport of gas phase components is important.
From the above results, it is considered that SiO ₂ nanofibers are formed by the mechanism of the reductive nitriding method, and further CH ₄ works as a reducing agent to form Si ₃ N ₄ .

  The present invention is not limited to the experimental examples described above, and various other configurations can be employed without departing from the gist of the present invention.

  As described above, the nanofiber according to the present invention has industrial applicability as a heat-resistant material having little influence on a living body.

Claims (1)

It is made of silicon nitride, has a diameter (fiber diameter) of 1 nm to 100 nm, is branched so that the nanofiber is branched into two, and the branched nanofiber is attached to another nanofiber at the branch destination. nanofibers having a network structure by.