Spinning solution for electrical forming, method for obtaining fibres by electrical forming, and fibres of silicone carbide

FIELD: electricity.

SUBSTANCE: spinning solution for electrical formation of polymer precursor of fibres of siliconecarbide contains 50 - 70 % solution of polycarbosilane with average molecular weight of 800 - 1500 astronomical units of weight, cross-linking agent and photoinitiator at the following molar ration of components: polycarbosilane/cross-linking agent/photoinitiator = 1/(0.5-1.5)/(0.5-2). Method for obtaining silicone carbide fibres involves preparation of spinning solution, electrical forming of fibres of precursor of silicone carbide from spinning solution with simultaneous cross-link of precursor fibres by light irradiation in visible or UV radiation range and heat treatment of precursor fibres for their conversion to silicone carbide fibres. Silicone carbide fibres made in compliance with the above method have average diameter of 50 nm to 2 mcm and porosity of less than 10 m2/g.

EFFECT: invention provides high capacity and low cost of production of high-quality silicone carbide fibres characterised with high mechanical strength and low porosity.

6 cl, 1 tbl

 

The invention relates to a technology for obtaining fibers of silicon carbide, in particular the production of micro - and nanofibers increased mechanical strength and chemical and thermal stability, which can find application in the production of high-temperature filtration and insulation materials, as well as reinforcing material in composite materials with a metal or ceramic matrix.

Recently has become fairly widespread methods of production of ceramic fibers as felt by electrotorture precursor fibers from a spinning solution based polymer compounds of silicon, in particular carbosilanes, followed by pyrolysis to convert the precursor into ceramic fibers, in particular fibers of silicon carbide.

So, in the international application WO 2009038767 disclosed method for the production of continuous ceramic fibers, including the production of polymeric ceramic precursor, which is used as polycarbosilane, its dissolution in a solvent to obtain a spinning solution, electropermanent from the solution of the continuous strands of polymeric precursor fibers of silicon carbide and the pyrolysis of the resulting strands of obtaining fibers of silicon carbide.

The closest technical solution is disclosed which international application WO 2008112755 (A1). In accordance with the technical solution provided in this application is disclosed a method of obtaining a ceramic filters, including on the basis of the fibers of silicon carbide, for use at high temperatures in corrosive environments. The process provides for the production of the spinning solution to electrotorture containing solution polycarbosilane in an organic solvent with additives, electropermanent mortar ceramic polymer precursor fibers with a diameter of from 10 nm to 1 μm and formed from precursor fibers on the collector Mat of such fibers. Then, this precursor Mat is subjected to pyrolysis and get a Mat of ceramic nanofibers of oxide and non-oxide ceramics diameter fibers, less than the precursor Mat. Fiber capable of operating at temperatures above 1000°C.

In all the above well-known technical solutions, conducting electrotorture assumes spinning solution, of at least one polymer characterized by either high molecular weight or highly branched structure, because only such a polymer can provide the necessary viscosity, surface tension and conductivity, optimal for electrotorture.

The disadvantages of these technical solutions are the high cost of the received fibers of silicon carbide, low productivity of the process, as well as the high porosity of the formed silicon carbide.

The objective of the invention is the elimination of all inherent in the known technical solutions deficiencies.

The problem is solved spinning solution for electrotorture polymeric precursor fibers of silicon carbide containing 50-70% solution of polycarbosilane corresponding to the formula

with an average molecular weight of 800-1500 u, a crosslinking agent and photoinitiator in the following molar ratio of the components: polycarbosilane / a crosslinking agent / photoinitiator=1/(0,5-1,5)/(0,5-2).

The task is also solved by a method of producing fibers of silicon carbide, comprising preparing the above-described spinning solution for electrotorture polymeric precursor fibers of silicon carbide, electropermanent fibers precursor of silicon carbide of the above-mentioned spinning solution with simultaneous curing of the precursor fibers by irradiation with light in the visible or ultraviolet radiation and thermal treatment of the precursor fibers to convert them into the fibers of silicon carbide.

In private embodiments of the invention the problem is solved by the fact that after electrotorture spend extra stitching exposure.

Possible crosslinking the neutral atmosphere or in a vacuum.

The task is also solved by the fibers of silicon carbide, which are characterized by the fact that is made in accordance with the above method, have an average diameter of from 50 nm to 2 μm and a porosity of at least 10 m2/year

The invention is carried out as follows.

The method involves the preparation of a solution of polycarbosilane (PCB) with a crosslinking agent and (optionally) photoinitiation, receiving fibers PKS method electrotorture with simultaneous cross-linking of fibers to the PCB by irradiation with light in the visible or ultraviolet ranges and subsequent heat treatment to convert the fibers to the PCB into the fibers of silicon carbide.

The essential feature of the invention is used as a precursor fibers of silicon carbide PCB corresponding to the formula

with an average molecular weight of 800-1500 u

Under polycarbosilane in the prior art is understood in a rather broad class of compounds of the General formula:

where R1-R4=Alk, H

(C)n=(-CH2-), -CH2-CH2-, (-CH2-)n (n>3),

-CH=, -CH=CH-, -C≡C-, - CH2 - C≡C - CH2 - arylene, xylylene and other

In our case, as it follows from the above formula (1), for the implementation of the invention selected those polycarbosilane, in which the side chains are methyl groups and hydrogen, the active site are methylene group is s, and the molecular weight of which is 800-1500 u

The solutions of these polycarbosilanes in organic solvents have an acceptable viscosity for the implementation of electrotorture, and during the pyrolysis of such carbosilane formed compounds of silicon and carbon, having a composition close to the stoichiometric composition of silicon carbide.

The advantage of using such polycarbosilanes (PKS) is not only that they are much cheaper than their counterparts with higher molecular weight, but also in achieving a higher concentration of silicon atoms in forming solutions while maintaining their viscosity suitable for electrotorture, because the viscosity of solutions of low molecular weight PCB increases with the concentration of PCB slower than solutions of high molecular weight PCB.

This makes it possible to increase the concentration of the solution of polycarbosilane to 70 wt.%, which leads to a lower solvent content in the molding solution and, consequently, reduces the porosity of the polymer fibers, caused by evaporation of the solvent, which allows to obtain a ceramic fiber with fewer pores.

The solvent may be used in any way acceptable for these purposes, a solvent, such as toluene, chloroform, dichloroethane, dichloroethylene, trichloroethylene, tetrachloroethylene, tetrahydrofuran, etc.

The disadvantages of data is of polimerov is that without adding to the spinning solution to electrotorture auxiliary substances in the claimed amount to provide a non-oxidizing cross-linking of the polymer, polycarbosilane cannot be obtained in the form of fibers by the method of electrotorture.

As a cross-linking agent used unsaturated hydrocarbon containing at least one double or triple C-C bonds, such as vinylbenzyl, divinylbenzene, diethylbenzene, 1,3-butadiene.

The choice of the molar ratio of the components polycarbosilane/a crosslinking agent=1/(0.5 to 1.5) depends on the number of unsaturated carbon bonds in the molecule cross-linking agent and due to the fact that when merging polymer participates in at least one unsaturated bond in the molecule cross-linking agent, so if there are ties in the cross-linking agent two, the crosslinking agent should be 0.5 mole of the crosslinking of one mole of the polymer.

The amount of crosslinking agent should be taken to excess, because not all bonds are involved in the stitching in mind steric and kinetic limitations of the crosslinking reaction. Typically, 50% of the excess cross-linking agent relative to polycarbosilane (i.e. the molar ratio of crosslinking agent to polycarbosilane is 1.5/1), the crosslinking of low molecular weight polycarbosilane most commercial cross-linking agents. Higher content of crosslinking AG the NTA can lead to excess carbon in the final product compared with the stoichiometric composition.

Introduction to spinning solution photoinitiator allows cross-linking of the polymer by irradiation with light in the visible and ultraviolet spectral ranges directly in the process of electrotorture fiber.

Such stitching provides the following advantages: 1) reduction of time-linkage compared to thermal curing; 2) reducing the oxygen content in the polymer precursor of silicon carbide compared to conventional oxidative curing, which increases the mechanical strength of the fibers of silicon carbide at high temperatures.

Under photoinitiation refers to compounds that convert the energy of electromagnetic radiation into chemical energy in the form of chemically active particles such as radicals or ions.

As photoinitiators can be used for a wide class of substances, such as aromatic monoketone (acetophenone, benzoin, benzophenone and derivatives thereof) and diketones (anthraquinone, phenanthridine, benzil and derivatives thereof), camphoroquinone, metallocene (ferrocene, titanocene) and others In the best embodiments of the invention is used 4,4'-bi(dimethylamino) benzophenone or 4-(dimethylamino) benzophenone.

Active particles generated by photoinitiation when the absorption of electromagnetic radiation of sufficient energy to cause the gap ninasimone the carbon-carbon links linking agent with the formation of active centers, which, interacting with two polymer molecules, and result merging. Therefore, the molar content of photoinitiator is determined by the number of unsaturated bonds in the cross-linking agent: if a crosslinking agent contains one unsaturated bond, photoinitiator must take at least 1 mole per mole of crosslinking agent; if a crosslinking agent contains two unsaturated communication, photoinitiator must take not less than 0.5 mole per mole of crosslinking agent. However, due to steric and kinetic limitations photoinitiator should be taken to excess. Typically, excess photoinitiator 1 mol sufficient to activate the vast amount of cross-linking agent and to ensure effective cross-linking of the polymer. The higher content of photoinitiator can lead to excess carbon in the final product compared with the stoichiometric composition.

The process of knitting, conducted by irradiation with light in the visible or ultraviolet radiation, is as follows: monofilament fiber or tow of fibers or felt from polycarbosilane exhibit in the stream of electromagnetic radiation of a visible or ultraviolet range of the spectrum of electromagnetic radiation must have a maximum intensity in the wavelength interval corresponding to the maximum in the spectrum of pogles the deposits photoinitiator; the exposure time is determined by the type of crosslinking agent, photoinitiator and irradiation dose and can last from a fraction of a second to several hours.

It should also be noted that cross-linking can occur at different stages of obtaining fibers of silicon carbide: in the process of electrotorture precursor fiber without additional crosslinks; implementation of additional crosslinks after electrotorture, for example before heat treatment of the precursor at its conversion into a ceramic fiber or heat treatment process. This may be implemented in various additional advantages of the invention.

So, joining conducted only in the process of electrotorture precursor fiber, allows you to save time on processing after their synthesis.

Additional stitching that is done after electrotorture precursor fiber, allows the best to remove any residual solvent from the fiber due to its exposure under reduced atmospheric pressure, which reduces the porosity of the fibers of the precursor at the stage of the Sol-gel transition in the polymer and, consequently, to reduce the porosity of the resulting ceramic fibers.

Additional stitching played on the stage of heat treatment, can further save time by combining the process of conversion of the precursor of the ceramic fiber process of polymer crosslinking, the use of low pressure gaseous environment in the process allows to obtain a ceramic fiber smallest porosity due to more complete removal of the solvent from the fibers of the precursor.

In the best embodiments of the invention, the joining is carried out in two stages - in the process of electrotorture and after of electrospinning, including at the stage of heat treatment of the precursor fiber, which allows to obtain a ceramic fiber with low porosity and with a minimum expenditure of time for processing of polymer fibers.

Heat treatment to convert the precursor (fiber (PKS)in the fibers of silicon carbide is carried out at temperatures of providing such conversion.

This treatment includes continuous heating to the temperature at which the transformation of polycarbosilane in silicon carbide, or step - by exposures at intermediate temperatures of heating.

A typical procedure pyrolysis stitched polycarbosilane disclosed in many sources, is as follows (inert environment of nitrogen, argon, helium):

1) heating from 300°C to 500°C at a rate of 2°C per minute and exposure 1 h;

2) heating from 500°C to 600°C at a rate of 1°C per minute and exposure 1 h;

3) heating from 600°C to 700°C at a rate of 1°C per minute and exposure 1 h;

4) heating from 700°C to 800°C at a rate of 1°C per minute and the per son who Rica 2 h;

5) heat up to 1200°C at a rate of 0.5-2°C per minute and extract 2-4 hours depending on thickness and porosity of the product.

After such heat treatment is obtained amorphous silicon carbide.

To obtain a crystalline silicon carbide conduct high-temperature crystallization of the amorphous silicon carbide according to the following procedure: heat at a rate of 2°C per minute in argon or helium to a temperature of not lower than 1600°C and incubated for 6-8 hours.

The degree of crystallinity of the obtained silicon carbide depends on the temperature and time of isothermal aging: at higher temperatures the time of isothermal exposure is reduced. The output of crystalline silicon carbide is 60-80% by weight of polycarbosilane depending on conditions knitting, pyrolysis and crystallization: the lower the heating rate and more time isothermal exposures at each stage of the heat treatment polycarboxyl when it is converted into silicon carbide, the greater the yield of crystalline silicon carbide.

Examples

1) Polycarbosilane (PKS), characterized by the formula

(average molecular weight MP~800-1500; unit weight: 1,03-1,08 g/cm3) was dissolved in an organic solvent is toluene, containing a crosslinking agent - diethenylbenzene and photoinitiator - 4,4'-bi(dimethylamino)benzophenone. Konz is trace PCB in solution was 70%; the molar ratio of the components: PLCs: (a crosslinking agent): photoinitiator=1,0:1,0:1,0.

2) Of the prepared solution method electrotorture conducted with electrical voltage 15-45 kV and the distance from the capillary to the precipitation electrode 15-30 cm, obtained fiber PCB in the form of a felt density of 0.1 g/cm3, whose thickness was 20±5 mm with an average diameter of the fibers is from 0.5 to 2 μm and 10±5 mm with an average diameter of the fibers ranges from 50 to 500 nm. Allowable thickness of the sample was determined by the effective penetration depth of light in the material, which depends on the thickness of the fibers and the density of the sample. The specified range of thickness is not limiting, but shows the most effective thickness of the samples for the specified interval densities of samples and average diameters of the fibers.

3) In the process of electrotorture obtained polymer felt was irradiated from a distance of 5-15 cm ultraviolet (UV) wavelength of 185 nm and the radiation flux at this wavelength is not less than 0.7 W within 1-2 h in air atmosphere or within 3-7 hours in a protective atmosphere of nitrogen, argon, helium or vacuum; in the latter case, use a vacuum chamber with Windows made from a material transparent to ultraviolet radiation, such as quartz.

These time intervals ultraviolet irradiation was determined by the type of cross-linking and the enta, the amount of crosslinking agent, the amount of photoinitiator, as well as the thickness of the sample, the average diameter of the fibers polycarbosilane and distance from the UV source to the sample: if the maximum content of the crosslinking agent and photoinitiator interval of concentrations specified for these components, when the thickness of the sample is not more than half of the maximum and minimum specified distance between the UV source and the sample exposure time corresponds to the minimum value of the specified interval.

4) After this polymer felt was subjected to heat treatment for converting fibers polycarbosilane in ceramic fiber. Modes such heat treatment depended on the conditions under which conducted the UV irradiation of a sample, in particular from the atmosphere. For felt, irradiation carried out in an inert atmosphere, heat treatment was carried out according to the following scheme:

1) heating in a weak stream of protective gas (nitrogen, argon) to 220°C at a rate not exceeding 1°C per minute;

2) aging at 220°C for 2 h;

3) heating up to 600°C at a rate not exceeding 1°C per minute;

4) exposure at 600°C for at least 1 h;

5) heating to 1600°C with a speed of not more than 3°C per minute;

6) exposure at 1600°C for at least 3 h;

7) cooling to room temperature.

For felt, irradiation which UV light was carried out in the stuffy atmosphere, the heating rate was increased to 2 times, and the times isothermal exposures, respectively, reduced in 2 times.

When such treatment is received fibers of crystalline silicon carbide, the content of amorphous phase of silicon carbide lower limit registration of x-ray phase method. The output of silicon carbide is 65 to 80% by weight of polycarbosilane depending on the type and concentration of crosslinking agent and photoinitiator.

The table shows the processing parameters and the resulting properties of the fibers of silicon carbide.

As follows from the presented data, the invention allows to obtain a high yield from inexpensive polycarbosilane high quality fiber silicon carbide, characterized by low porosity.

1. The spinning solution to electrotorture polymeric precursor fibers of silicon carbide, characterized in that it contains 50-70%solution of polycarbosilane corresponding to the formula

with an average molecular weight of 800-1500 u, a crosslinking agent and photoinitiator in the following molar ratio of the components: polycarbosilane/a crosslinking agent/photoinitiator = 1/(0,5-1,5)/(0,5-2).

2. The method of producing fibers of silicon carbide, characterized in that it includes the preparation of the spinning solution is La electrotorture polymeric precursor fibers of silicon carbide in accordance with claim 1 of the formula, electropermanent fibers precursor of silicon carbide of the above-mentioned spinning solution with simultaneous curing of the precursor fibers by irradiation with light in the visible or ultraviolet radiation and thermal treatment of the precursor fibers to convert them into the fibers of silicon carbide.

3. The method according to claim 2, characterized in that after electrotorture spend extra stitching exposure.

4. The method according to claim 2, characterized in that the stitching is carried out in a neutral atmosphere.

5. The method according to claim 2, characterized in that the stitching is carried out in a vacuum.

6. Fibers of silicon carbide, characterized by the fact that they are made in accordance with any of claim 2 to 5 formulas that have an average diameter of from 50 nm to 2 μm and a porosity of at least 10 m2/,



 

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