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HIGHLY ORIENTED NANOMETER MAX PHASE CERAMIC AND PREPARATION METHOD FOR MAX PHASE IN-SITU AUTOGENOUS OXIDE NANOCOMPOSITE CERAMIC

20210317045 · 2021-10-14

    Inventors

    Cpc classification

    International classification

    Abstract

    A highly oriented nanometer MAX phase ceramic and a preparation method for a MAX phase in-situ autogenous oxide nanocomposite ceramic. The raw materials comprise a MAX phase ceramic nano-lamellar powder body or a blank body formed by the nano-lamellar powder body, wherein MAX phase ceramic nano-lamellar particles in the powder body or the blank meet the particle size being between 20-400 nm, and the oxygen content is between 0.0001%-20% by mass; MAX phase grains in the ceramic obtained after the raw materials are sintered are lamellar or spindle-shaped, the lamellar structure having a high degree of orientation. Utilizing special properties of the nano-lamellar MAX powder body, orientation occurs during compression and deformation to obtain a lamellar structure similar to that in a natural pearl shell, and such a structure has a high bearing capacity and resistance to external loads and crack propagation, just like a brick used in a building.

    Claims

    1. A method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics, characterized in that, details as follows: (1) preparing a raw material which is made of MAX phase ceramic nanosheet layered powder or embryo body formed by nanosheet layered powder, nanosheet layered particles of MAX phase ceramics in the powder or embryo body meet a particle size of 20-400 nanometers, and an oxygen content of 0.0001%-20% by mass fraction; (2) after sintering the raw materials, obtaining a ceramic of which MAX phase crystal grains in the ceramic are lamellar-shaped or spindle-shaped, and sheet layers of the ceramic are highly oriented.

    2. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: the nano MAX phase ceramics are distributed in an orderly stack of bricks and MAX phase oxides are distributed at grain boundaries of the nano MAX phase ceramic grains, and a MAX phase grain size is 20-400 nm.

    3. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: in terms of mass percentage, in the nano MAX phase ceramics and the MAX phase oxide nanocomposite ceramics, a content of nano MAX phase oxide is 0.0002%-40%, and the rest is nano MAX phase ceramics.

    4. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: a sintering method which utilizes nano powder or embryo body directly for sintering with pressure, or a sintering method which utilizes nano powder or embryo body directly for pre-compression molding followed by sintering without pressure is employed.

    5. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: the sintering method which utilizes nano powder or embryo body directly for sintering with pressure is carried out by using a hot pressing sintering process, a hot isostatic pressing sintering process or a spark plasma sintering process, wherein: (1) in the hot pressing sintering process: the nanosheet layered powder or the embryo body is directly loaded into a graphite mold, and hot pressing sintering is carried out with the powder or the embryo body inside the graphite mold, a sintering temperature is 500-2000° C., a sintering pressure is 1-200 MPa, a holding time is 10-3600 minutes, and a heating rate is 1-100° C. per minute, the sintering process is carried out under vacuum or argon atmosphere; (2) in the hot isostatic pressing sintering process: the nanosheet layered powder or embryo body directly is put into a hot isostatic pressing jacket, and then the jacket is vacuumed and sealed; the hot isostatic pressing sintering process is carried out with the powder or the embryo body inside the jacket, the sintering temperature is 500-2000° C., the sintering pressure is 1-800 MPa, the holding time is 10-3600 minutes, and the heating rate is 1-100° C. per minute, the sintering process is carried out under vacuum or argon atmosphere; (3) in the spark plasma sintering process: the nanosheet layered powder or embryo body is put directly into a sintering mold, and a large pulse current is applied for the sintering process, the sintering temperature is 300-1800° C., the sintering pressure is 1-400 MPa, the holding time is 5-600 minutes, and the heating rate is 1-500° C. per minute, the sintering process is carried out under vacuum or argon atmosphere.

    6. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: the sintering method which utilizes nano powder or embryo body directly for pre-compression molding followed by sintering without pressure, comprising one of the followings: (1) put the nano powder or the embryo body into a pressing mold, apply pressure to the mold to process densification, a pressure applied is 5-1000 MPa, and then obtain a compressed product to process sintering without pressure; (2) put the nano powder or the embryo body into a cold isostatic pressing jacket, and then vacuum and seal the jacket; process cold isostatic pressing sintering with the powder or the embryo body inside the jacket for densification, a cold isostatic pressing temperature is 300-1800° C., a cold isostatic pressing pressure is 1-800 MPa, a holding time is 10-3600 minutes, and a heating rate is 1-100° C. per minute, then obtain a compressed product from the jacket to process sintering without pressure; (3) put the nano powder or the embryo body into a jacket or use the embryo body directly to carry out rolling, a rolling pressure is 10-1000 MPa, a rolling temperature is 0-600° C., and then obtain a molded product of MAX phase ceramic nanosheet-layered powder or embryo body after rolling to process sintering without pressure; (4) use the obtained pre-compressed formed MAX phase ceramic nanosheet-layered product to process sintering, the sintering method is: put the powder into a container that can withstand a sintering temperature, and then vacuum the container or pass protective gas, or put the powder directly into a furnace body that is vacuumed or passed with protective gas to process sintering without pressure.

    7. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 6, characterized in that: an equipment used for sintering is muffle furnace, induction heating furnace, microwave heating furnace, and infrared heating furnace, the sintering temperature is 300-2000° C. and the sintering time is 10-9600 minutes.

    8. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according to claim 1, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    9. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 2, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    10. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 3, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    11. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 4, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    12. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 5, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    13. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 6, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    14. The method for preparing highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics according claim 7, characterized in that: when preparing an oriented MAX phase/oxide nanocomposite ceramics, an oxide content in the MAX phase/oxide nanocomposite ceramics is controlled by an oxygen content of the nanosheet-layered powder or the embryo body used to prepare the MAX phase ceramics, the obtained crystal grain size is controlled by a particle size of the nanosheet-layered particles and powder sintering parameters, and a degree of orientation of the obtained ceramic is controlled by different combinations and parameters of the pressing method and the sintering method.

    Description

    DESCRIPTION OF THE DRAWINGS

    [0035] FIG. 1 is a pole figure obtained by electron backscatter diffraction (EBSD) characterization on the pressure surface of the final sintered specimen. (a) is the pole figure of the crystal plane 001 in the Ti.sub.2AlC phase crystal in the Ti.sub.2AlC/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane; (b) is the pole figure of the 11-20 crystal plane in the Ti.sub.2AlC phase crystal in the Ti.sub.2AlC/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane; (c) is the pole figure of the 10-10 crystal plane in the Ti.sub.2AlC phase crystal in the Ti.sub.2AlC/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane.

    [0036] FIG. 2 is the structure image obtained by electron backscatter diffraction characterization on the vertical pressure surface of the final sintered sample.

    [0037] FIG. 3 illustrates X-ray crystal diffraction data of parallel pressure surface and vertical pressure surface of the final sintered sample. In the figure, the abscissa ‘2 Theta’ represents the diffraction angle (deg.), and the ordinate ‘Intensity’ represents the intensity; ‘1-vertical surface’ represents the vertical pressure surface, and ‘2-parallel surface’ represents the parallel pressure surface.

    [0038] FIG. 4 is a pole figure obtained by electron backscatter diffraction (EBSD) characterization of the pressure surface of the final sintered specimen. (a) is the pole figure of the crystal plane 001 in the Ti.sub.3AlC.sub.2 phase crystal in the Ti.sub.3AlC.sub.2/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane; (b) is the pole figure of the 11-20 crystal plane in the Ti.sub.3AlC.sub.2 phase crystal in the Ti.sub.3AlC.sub.2/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane; (c) is the pole figure of the 10-10 crystal plane in the Ti.sub.3AlC.sub.2 phase crystal in the Ti.sub.3AlC.sub.2/Al.sub.2O.sub.3 nanocomposite ceramic on the diffraction projection plane.

    [0039] FIG. 5 is the structure image obtained by electron backscatter diffraction characterization on the vertical pressure surface of the final sintered sample.

    DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION

    [0040] In the implementation process of the embodiment, a preparation method for highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics of the present invention is as follows:

    [0041] (1) Prepare a raw material which is made of MAX phase ceramic nanosheet layered powder or embryo body formed by nanosheet layered powder, the nanosheet layered particles of MAX phase ceramics in the powder or embryo body meet the particle size between 20˜400 nanometers (preferably 100˜200 nanometers), and the oxygen content between 0.0001%˜20% by mass fraction (preferably 0.02%˜10%).

    [0042] (2) After sintering the raw materials, obtain a ceramic in which a MAX phase crystal grains in the ceramic are lamellar-shaped or spindle-shaped, and the sheet layers of the ceramic are highly oriented.

    [0043] (3) The preparation method can utilize nano powder or embryo body directly for sintering with pressure. For example, a hot-pressing sintering method is employed: the nanosheet layered powder or the embryo body is directly loaded into a graphite mold, and is hot-pressed sintered inside the graphite mold, the sintering temperature is 500˜2000° C., the sintering pressure is 1˜200 MPa, the holding time is 10˜3600 minutes, and the heating rate is 1˜100° C. per minute, the sintering process is carried out under vacuum or argon atmosphere. The hot isostatic pressing sintering method is employed: the nanosheet layered powder or embryo body is put directly into a hot isostatic pressing jacket, and then the jacket is vacuumed and sealed. The nanosheet layered powder or embryo body is sintered inside the jacket by hot isostatic pressing sintering, the sintering temperature is 500˜2000° C., the sintering pressure is 1˜800 MPa, the holding time is 10˜3600 minutes, and the heating rate is 1˜100° C. per minute, the sintering process is carried out under vacuum or argon atmosphere. The spark plasma sintering method is employed: the nanosheet layered powder or embryo body is put directly into a sintering mold and is sintered under the application of a large pulse current, the sintering temperature is 300˜1800° C., the sintering pressure is 1˜400 MPa, the holding time is 5˜600 minutes, and the heating rate is 1˜500° C. per minute, the sintering process is carried out under vacuum or argon atmosphere. The sintering with pressure with nano powder or embryo body is not limited to the methods as listed above. Any sintering method with pressure that can apply external effects to the powder to deform and sinter at the same time is also within the protection scope of the present invention.

    [0044] (4) The sintering method which utilizes the nano powder or the embryo body directly for pre-compression molding, followed by sintering without pressure, is employed. For examples: put the nano powder or embryo body into a pressing mold, apply pressure to the mold to process densification, the pressure applied is 5˜1000 MPa, and then use the obtained compressed product to carry out sintering without pressure. Besides, the nano powder or embryo body can also be put into the cold isostatic pressing jacket, and then vacuum and seal the jacket; process cold isostatic pressing sintering of the nano powder or embryo body inside the jacket for densification, the cold isostatic pressing temperature is 300˜1800° C., the cold isostatic pressing pressure is 1˜800 MPa, the holding time is 10˜3600 minutes, and the heating rate is 1˜100° C. per minute. Then take out the compressed product from the jacket to process sintering without pressure. Moreover, the nano powder or the embryo body can be put into a jacket or the embryo body can be used directly to carry out rolling, the rolling pressure is 10˜1000 MPa, the rolling temperature is 0˜600° C., and then use the obtained molded product of MAX phase ceramic nanosheet-layered powder or the embryo body after rolling to process sintering without pressure. The nano-powder or embryo body with pre-compression molding is not limited to the methods listed above, and any pressurizing method that can apply external effects to the powder to deform is also within the protection scope of the present invention. The method of sintering without pressure can be performed on the obtained pre-compression molding of MAX phase nanosheet layered products. The sintering method can be as follows: put the powder into a container that can withstand the sintering temperature, and then vacuum the container or pass protective gas (such as argon gas), or put the powder directly into a furnace body that is vacuumed or passed with protective gas (such as argon gas) for sintering without pressure. The equipment used for sintering can be any equipment that can heat the sample to process sintering and densification, such as muffle furnace, induction heating furnace, microwave heating furnace, and infrared heating furnace. The sintering temperature is 300˜2000° C. and the sintering time is 10˜9600 minutes. The method of sintering without pressure on the pre-compression molding of nano powder is not limited to the methods listed above. Any sintering method that can apply a temperature field to the powder is within the protection scope of the present invention.

    [0045] (5) When preparing oriented nano MAX phase ceramics and MAX phase/oxide nano composite ceramics by the above method, in particular, whether it is nano MAX phase ceramics or MAX phase/oxide nanocomposite ceramics, and the oxide content in MAX phase/oxide nanocomposite ceramics, the oxygen content is adjusted and controlled by the oxygen content of the nanosheet layered powder or the embryo body of MAX phase ceramics used to prepare the above ceramics, the obtained crystal grain size is controlled by the particle size of the nanosheet particles and the powder sintering parameters, the degree of ceramic orientation obtained is jointly controlled by the different combinations and parameters of the pressing method and the sintering method in the preparation method.

    [0046] The present invention is further described in detail with the embodiments and accompanying drawings as following:

    Embodiment 1

    [0047] According to this embodiment, a preparation method of highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics is as follows:

    [0048] Weigh 200 grams of MAX phase ceramic nanosheet layered powder named Ti.sub.2AlC, the particle size of the powder is 180 nanometers, and the oxygen content of the powder is 8% by mass fraction. The nanosheet layered powder is directly loaded into a graphite mold, and hot pressing sintering method is employed. The hot-pressed sintered is carried out with the powder inside the graphite mold, the sintering temperature 1250° C., the sintering pressure is 50 MPa, the holding time is 60 minutes, and the heating rate is 5° C. per minute, and the sintering process is carried out under vacuum condition. After the sintering process, Ti.sub.2AlC/Al.sub.2O.sub.3 nano-composite ceramics is obtained, and the alumina content accounts for 12% of the material by mass fraction. The compressive strength of the material is 2200 MPa, which is much higher than the 400˜1000 MPa strength of ordinary Ti.sub.2AlC, and the fracture toughness is 8-9 MPa.Math.m.sup.1/2, which is much higher than the 5-6 MPa.Math.m.sup.1/2 fracture toughness value of ordinary Ti.sub.2AlC, and the high temperature performance is: a compressive strength reaches 400 MPa at 1000° C.

    [0049] As shown in FIG. 1, the pole figure obtained by electron backscatter diffraction (EBSD) characterization on the pressure surface of the final sintered sample is illustrated. It can be seen from this figure that the direction of the crystal plane 001 of the Ti.sub.2AC phase is almost completely concentrated in the center of the projection plane, and the direction of the crystal plane 11-20 and the direction of the crystal plane 10-10 are almost completely parallel to the projection plane, and this shows the orientation of the material.

    [0050] As shown in FIG. 2, the structure image obtained by electron backscatter diffraction characterization on the vertical pressure surface of the final sintered sample is illustrated. It can be seen from this figure that the material structure and grains have obvious orientation.

    [0051] As shown in FIG. 3, the X-ray crystal diffraction data of parallel pressure surface and vertical pressure surface of the final sintered specimen is illustrated. It can be seen from the figure that the main crystal planes of the Ti.sub.2AC phase parallel to the pressure surface are (001) crystal planes including (002) and (006) crystal planes. The peak position of the crystal plane (001) of the Ti.sub.2AC phase perpendicular to the pressure surface basically disappears. The main crystal planes are crystal planes (101) such as crystal planes (101), (110), (103), and etc. It shows obvious orientation of the material in different directions.

    [0052] According to this embodiment, in the ceramic obtained after sintering the raw materials, the MAX phase crystal grains are lamellar-shaped or spindle-shaped, the nano MAX phase ceramics are distributed in an orderly stack of bricks and MAX phase oxides are distributed at the grain boundaries of the nano MAX phase ceramic grains. The MAX phase grain size is 50˜300 nanometers in thickness and 0.5˜3 microns in width.

    Embodiment 2

    [0053] According to this embodiment, a preparation method of highly oriented nano MAX phase ceramics and MAX phase in-situ self-generating oxide nano-composite ceramics is as follows:

    [0054] Weigh 500 grams of MAX phase ceramic nanosheet layered powder named Ti.sub.3AlC.sub.2, the particle size of the powder is 200 nanometers, and the oxygen content of the powder is 5% by mass fraction. The nanosheet layered powder is put directly into a stainless steel jacket, and the jacket is vacuumed and sealed. The sealed jacket is put into a hot isostatic pressing furnace for sintering. The sintering temperature is 1100° C., the sintering pressure is 200 MPa, the holding time is 120 minutes, and the heating rate is 5° C. per minute, and the sintering atmosphere is argon gas. After the sintering process, Ti.sub.3AlC.sub.2/Al.sub.2O.sub.3 nano-composite ceramics is obtained, and the alumina content accounts for 8% of the material by mass fraction. The compressive strength of the material is 1800 MPa, and the fracture toughness is 14-17 MPa.Math.m.sup.1/2, which is much higher than the 7-8 MPa.Math.m.sup.1/2 fracture toughness value of ordinary Ti.sub.3AlC.sub.2, and the high temperature performance is: a compressive strength reaches 350 MPa at 1000° C.

    [0055] As shown in FIG. 4, the pole figure obtained by electron backscatter diffraction (EBSD) characterization on the pressure surface of the final sintered sample is illustrated. It can be seen from this figure that the direction of the crystal plane 001 of the Ti.sub.3AlC.sub.2 phase is almost completely concentrated in the center of the projection plane, and the direction of the crystal plane 11-20 and the direction of the crystal plane 10-10 are almost completely parallel to the projection plane, and this shows the orientation of the material.

    [0056] According to this embodiment, in the ceramic obtained after sintering the raw materials, the MAX phase crystal grains are lamellar-shaped or spindle-shaped, the nano MAX phase ceramics are distributed in an orderly stack of bricks and MAX phase oxides are distributed at the grain boundaries of the nano MAX phase ceramic grains. The MAX phase grain size is 100˜400 nanometers in thickness and 1˜10 microns in width.

    [0057] As shown in FIG. 5, the structure image obtained by electron backscatter diffraction characterization on the vertical pressure surface of the final sintered sample is illustrated. It can be seen from the image that the material structure and crystal grains have obvious orientation, and the lamellar structure of the material is very obvious and complete.

    Embodiment 3

    [0058] Weigh 1 kg of MAX phase ceramic nanosheet layered powder named Ti.sub.3SiC.sub.2, the oxygen content of the powder is 6% by mass fraction, and the particle size of the powder is 80 nanometers.

    [0059] The nanopowder is put into an aluminum alloy cold isostatic pressing jacket, then the jacket is vacuumed and sealed, and process cold isostatic pressing and the nanopowder is densified inside the jacket, the cold isostatic pressing temperature is 400° C., and the cold isostatic pressing pressure is 250 MPa , the pressure holding time is 360 minutes, and the heating rate is 5° C. per minute. Then the pressed powder product is taken out of the jacket. Put the pressed embryo body into an alumina crucible and send it to a vacuum furnace for sintering at a sintering temperature of 1300° C. and a sintering time of 180 minutes. After the sintering process, Ti.sub.3AlC.sub.2/SiO.sub.2 nano-composite ceramics is obtained, and the silica content accounts for 10% of the material by mass fraction. The compressive strength of the material is 1900 MPa, and the fracture toughness is 12˜15 MPa.Math.m.sup.1/2, which is much higher than the 7˜8 MPa.Math.m.sup.1/2 fracture toughness value of ordinary Ti.sub.3SiC.sub.2, and the high temperature performance is: a compressive strength reaches 320 MPa at 1000° C.

    [0060] According to this embodiment, in the ceramic obtained after sintering the raw materials, the MAX phase crystal grains are lamellar-shaped or spindle-shaped, the nano MAX phase ceramics are distributed in an orderly stack of bricks and MAX phase oxides are distributed at the grain boundaries of the nano MAX phase ceramic grains. The MAX phase grain size is 100˜400 nanometers in thickness and 1˜10 microns in width.