Preparation and application of boron nitride ceramic fiber
Author: lizongpeng, Liang bingning, Zhigao, Wang Changshong
Source: Contemporary Chemical Engineering, 2017, issue 07
Abstract: boron nitride ceramic fiber is a new developing high-performance material. However, it is difficult to prepare high-quality boron nitride ceramic fiber materials by traditional high-temperature method, which can only be achieved by precursor conversion method. The synthetic routes of boron nitride ceramic fibers and the advantages and disadvantages of various precursor methods for preparing boron nitride ceramic fibers are summarized, and the development trend of precursor method for preparing boron nitride ceramic fibers is prospected.
Key words: boron nitride; Ceramic fiber; Precursor
CLC classification number: TQ 343.7 document identification code: a article number: 1671-0460 (2017) 07-1453-05
Preparation and Application of Boron Nitride Ceramic Fiber
LI Zong-peng， LIANG Bing， NING Zhi-gao， WANG Chang-song
（College of Materials Science and Engineering， Shenyang University of Chemical Technology， Liaoning Shenyang 110142，China）
Abstract： Boron nitride ceramic fiber is a new developing high-performance material. But the traditional method of high-temperature synthesis is hard to prepare high-quality boron nitride ceramic fiber which can only be prepared by the method of polymer derived ceramics（PDCs）. In the article， synthesis methods of boron nitride ceramic fiber were introduced as well as the advantages and disadvantages of boron nitride ceramic fibers prepared with various precursors. At last， the prospect and trend of research of polymer derived boron nitride ceramic fiber were viewed.
Key words： Boron nitride； Ceramic fiber； Precursor
Boron nitride (BN) is an important non oxide ceramic material, mainly including α- BN， β- BN， γ- BN has three crystal structures, among which, α- BN， γ- BN is hexagonal crystal form, collectively known as h-BN. Its layered crystal structure is similar to that of graphite. Each layer of h-BN crystal structure is composed of b3n3 six membered ring units, showing obvious anisotropy . BN is a kind of high temperature resistant crystal with white lubricity. Its insulation performance is particularly good. Even at high temperature, its insulation performance exceeds that of the most high-temperature resistant oxide. Its resistivity is 1900 Ω • cm at 2000 ℃, and >012 Ω • cm at room temperature. Its coefficient of thermal expansion is as low as -2.9 × 10-6/deg。 It does not soak with a variety of metals, has excellent corrosion resistance, and has small dielectric constant and loss tangent. These excellent properties make BN materials widely used in ceramics, coatings, aerospace engineering and many other industrial fields [3, 4]. Boron nitride fiber has the advantages of both BN material and fiber material. It has a good application prospect in composites, aerospace, nuclear industry and high temperature filter materials. Boron nitride fiber has the characteristics of low density, high strength, oxidation resistance, excellent dielectric properties, good thermal conductivity, good compatibility, strong wave transmission, etc. Therefore, as a high-performance fiber material, the preparation method of BN fiber is the current research hotspot [5-8].
Due to the strong bond energy between B atom and N atom, B atom is difficult to diffuse in its nitride, and it is difficult to prepare nitride ceramic fibers with excellent properties by traditional powder metallurgy. Therefore, this kind of ceramic fibers can be effectively prepared only by polymer precursor conversion (pDCs) . The temperature required for the conversion of polymeric derivatives into inorganic ceramics is low, and new ceramic materials with diverse and complex structures that are difficult to prepare by traditional methods can be obtained, and polymer precursors with different structures can be obtained through molecular design . At present, pDCs method has become the main method used to develop high-performance BN fibers. Foreign research groups have made preliminary achievements here, and domestic related research work has just begun.
1 preparation method
In 1976, Japanese researcher taniguchi first reported the method of preparing BN fiber by organic precursor method. BN fiber was prepared by pyrolysis with n-phenyl-aminoborazine monomer as precursor. The obtained BN fiber has good tensile strength. Since then, many research groups have carried out research on the preparation of BN fibers by organic precursor method. At present, the main monomers and synthetic routes used to prepare polycyclic borazine precursors are divided into the following categories.
1.1 taking cycloborazane as monomer
Cycloborazane (h3b3n3h3, BZ) is a liquid molecule. Its first synthesis was realized by stock in 1926. It has appropriate b/n atomic ratio, good symmetry and the highest theoretical yield of BN Ceramics.
In the 1990s, Sneddon et al.  first used heat initiated dehydrogenation coupling reaction, heated to 70 ℃ in vacuum to react between N-H and B-H to obtain polycyclic borazane. Its weight average molecular weight (MW) is 7600, and the yield of ceramics heated to 1200 ℃ in argon atmosphere is 85% - 93%. However, the reactivity of B-H and N-H on BZ molecule is strong, and it is easy to form a highly crosslinked network structure, which usually prevents it from forming a crystalline boron nitride structure in the liquid or melting process.
Dong s, et al.  of the Chinese Academy of Sciences used NaBH4 to reduce trichloro cycloborazane (TCB) to obtain a mixture of cycloborazane, monochloro cycloborazane and dichlorocycloborazane. After NH3 was added for a period of time, an oily oligomer precursor was obtained. The reaction principle is shown in Figure 1. Its number average molecular weight (MN) is 860 and the degree of polymerization is about 9. Solidified and cracked at high temperature in ammonia and argon atmosphere, a high crystallinity layered boron nitride crystal with grain diameter of 50 ~ 100 nm was obtained. It has good oxidation resistance in air.
In order to reduce the crosslinking degree of polycyclic borazene and synthesize precursors more suitable for processing, various research groups modified the structure of the precursors. Sneddon research group [14, 15] reduced the amount of B-H and N-H in its structure and controlled the dehydrogenation coupling reaction by introducing DIALKYLAMINE grafted on polycyclic borazine. The machining performance is improved, and the diameter is 30 μ M fiber, but the mechanical properties are poor, and the strength is only 0.18 GPa. Lynch group [16, 17] polymerized by introducing unsaturated hydrocarbons on the B atom of cycloborazane to obtain polyb vinyl cycloborazane with six membered rings of B and N in the side chain. It provides a new idea for preparing BN fiber.
Although the introduced side groups can be used as plasticizers to improve the processability of polycyclic borazine, the lack of flexible bond connection in its structure limits the ability to prepare high-performance fibers. Therefore, this route is more suitable for synthesizing BN powder, coating, lubricant and composite matrix .
1.2 trichloro cycloborazane as monomer
Due to the lack of selectivity of cycloborazane, its application as BN precursor is limited. Therefore, most researchers have focused on the synthesis of b-trichloro cycloborazane (TCB) monomer through boron trichloride and ammonium chloride under certain conditions [19, 20]. Trichloro cycloborazane has higher thermal stability than cycloborazane. Through its nucleophilic substitution and cross-linking deamination reaction, poly [b- (alkylamino) cycloborazane] and poly [(borane amino) cycloborazane] are generated. On the basis of the repeated b3n3 ring, the obtained polymer is mainly connected between the cycloborazane rings by organic groups -nr- or -nh- which effectively improves the processability of the precursor.
Polymerization requires the nucleophilic substitution reaction of chlorine atoms to bond the part to be connected with boron atoms. In fact, the overall synthesis path can be divided into one-step and two-step reactions, and the synthesized polymer depends on the activity of nucleophiles.
1.2.1 one step synthesis method
When the nitrogen-containing nucleophile reagent is highly active at low temperature, a monomer compound will be obtained, and the self condensation of the monomer will produce a polymer precursor with low crosslinking density.
The one-step synthesis of polymers using trichloro cycloborazane and hexamethyldisilazane (hmdz) has been studied by many researchers. This reaction takes place based on the formation of stable and volatile me3sicl, which acts like a driving force.
Narula et al.  first used this method to synthesize the polyamino cycloborazine precursor, which can be dissolved in common organic solvents, but is not fusible. It may be that hexamethyldisilazane has a substitution reaction with three CL in TCB at the same time, and the crosslinking degree increases. The researchers pointed out that the polymer is a good precursor for preparing porous or thin-film BN materials with high ceramic yield. In order to prepare linear precursors, dimethylamine can be used to replace one Cl atom on TCB in advance and react with hmdz in the mechanism of the above reaction .
1.2.2 two step synthesis method
As a general two-step process, the chlorine atom on trichloro cycloborazane is first replaced by ammonia or amine nucleophile, and the reaction is carried out in the presence of tertiary amine (such as et3n). These amines or ammonia are connected to the B atom, and the released hydrogen chloride precipitates out in the form of salt (et3n • HCl). The obtained amino cycloborazane undergoes amination reaction to produce the corresponding polyamino cycloborazane. B. N six membered rings are connected by -nh- or -nr- .
Kimura et al.  and duperrier et al.  used this route to synthesize trimethylamino cycloborazane, which was self condensed in a nitrogen atmosphere above 200 ℃ to form polytrimethylamino cycloborazane, which was mainly entangled with -n (CH3) - as a bond bridge. In order to reduce the crosslinking formed in the thermal decomposition process, Kimura et al.  introduced laurylamine (c12h25nh2) in the thermal polymerization process to improve the plasticity of the polymer, prevent excessive crosslinking, improve the processability of the precursor, and obtain a diameter of 10 μ m. BN fiber with tensile strength of 980 MPa and Young's modulus of 78 GPa.
Meanwhile, duperrier et al. [26, 27] studied the effects of different pyrolysis temperatures on the properties of polymer precursors. The results showed that the glass transition temperature (TG) of the precursor increased with the increase of pyrolysis temperature. At low temperature（
In order to further improve the spinning performance of the precursor. Toury et al.  used dimethylamine / methylamine to replace TCB step by step to obtain different amine substituted derivative monomers. The spinning properties of the precursors polymerized with different monomers were studied respectively. Among them, with 2-dimethylamino-4,6-bis (methylamino) cycloborazane as monomer, the precursor with weight average molecular weight of 840 ~ 1000, glass transition temperature of 60 ~ 90 ℃, and ceramic yield of 50.6 ~ 54.7% at 1000 ℃ can be obtained by changing the pyrolysis conditions. The precursor is mainly connected by B-N bond and -n (CH3) - bond between rings. In addition, it is also found that there is an inter ring -n (H) - bond. It is suggested that the internal circulation rearrangement reaction may be caused by the secondary amino substituent connected to the N atom in the monomer. The reaction principle is shown in Figure 3. The existence of B-N bond between rings makes it have better chemical stability than polytrimethylamino cycloborazane (pmab). The diameter of 25.2 can be obtained by melt spinning μ The BN ceramic fiber with a diameter of 11.2 can be obtained by almost complete crystallization at 1600 ℃ μ m. The tensile strength is 1.18 GPA and the young's modulus E is 193 GPa.
Domestic Deng Cheng et al.  used the steric hindrance effect of n (isopropylamine) to make the substitution reaction between n (isopropylamine) and Cl atom in TCB molecule respectively. The effects of different amino substituents in the monomer on the thermal polymerization properties of the precursor and the structure and processability of the polymerization products were discussed. The results show that isopropylamino has strong thermal polymerization, and it is easy to crosslink to form a network structure of non molten solid when heated, while n-propylamino is difficult to thermal polymerization, showing thermal polymerization inertia. Therefore, when the molar ratio of n-propylamine / isopropylamine is controlled to be 2:1, cosubstitution is carried out to obtain an average monomer with 1 n-propylamino and 2 isopropylamino attached to one cycloborazine molecule. The number average molecular weight (MN) and weight average molecular weight (MW) of polycyclic borazane obtained by polymerization of the monomer at 150 ℃ are 709 and
1252, and the dispersion coefficient is 1.764. The melting point is 90 ℃. It has certain spinnability, and the diameter of melt spinning is about 15 μ M precursor fiber, with uniform fiber diameter and a small amount of particulate impurities on the surface. The surface of boron nitride fiber calcined at 1200 ℃ is smooth and dense, the carbon content is only 0.17%, and the ceramic yield is 68.4%. At the same time, the different reactivity of methylamine / dimethylamine is also used [30, 31]. The thermal polymerization properties of methylamino / dimethylamino precursors and their effects on the structure and processability of the products were discussed. The research shows that methylamine is more likely to react with Cl atom in TCB, while dimethylamine has low reaction activity and shows chemical inertia. Therefore, when the molar ratio of methylamine / dimethylamine is controlled to be 1:2, the substituted monomer is heated to 180 ℃ and polymerized. The number average molecular weight (MN) of polycyclic borazane is 191, the weight average molecular weight (MW) is 2152 and the dispersion coefficient is 1.80. The melting point is 83 ℃, which can be dissolved in benzene, toluene and other organic solvents. The yield of ceramics heated to 1000 ℃ in nitrogen atmosphere is 57.3%. Its molecular structure is approximately one-dimensional linear arrangement, and it has excellent spinnability. The diameter of molten pick wire is about 10 ~ 15 μ M precursor fiber, with uniform fiber diameter, smooth surface and dense structure.
Lei Yongpeng et al.  of the University of defense technology further studied the step-by-step substitution of TCB by n-propylamine / Methylamine on this basis. By strictly controlling the reaction conditions, n-propylamine was first substituted for a Cl atom in TCB molecule to block the end, and the intermediate obtained was then reacted with methylamine. Finally, the monomer containing one inert group and two active groups is obtained, which is conducive to the thermal polymerization reaction in the linear direction, and the precursor with excellent spinnability is obtained. The results show that the number average molecular weight (MN) of the precursor obtained by this method is 1002, the weight average molecular weight (MW) is 1359, and the dispersion coefficient is 1.5. It is softened at about 95 ℃, soluble in solvents such as toluene and xylene, but insoluble in chloroform and tetrahydrofuran. When heated to 1000 ℃ in argon atmosphere, the ceramic yield is about 50%. Melt spinning can obtain a smooth surface with a diameter of about 20 μ M. After treatment, the density of BN fiber is 1.92 g • cm-3, the tensile strength is 850 MPa, the carbon content is less than 0.5% (WT), the dielectric constant is about 3, and the oxidation resistance is good.
Zhong B et al.  of Harbin Institute of technology used liquid aniline / anisidine as amine source, and carried out step-by-step substitution reaction reheat polymerization with the ratio of TCB: anisidine: aniline =1:1.5:1 under certain conditions to obtain Polymerized Polyaniline cycloborazane
(poly[(phenylamine) borazine], the reaction principle is shown in Figure 4. The number average molecular weight (MN) of the precursor is 30520 and the degree of polymerization is 319. High temperature pyrolysis at 1200 ℃ in ammonia atmosphere for 2 hours to obtain fibers with uniform diameter, dense and defect free, with a diameter of about 3 ~ 4 μ m。 The reaction is characterized by the use of amine sources such as aniline and anisidine, which makes the aminolysis substitution reaction a homogeneous liquid-phase reaction. Make the reaction process more controllable.
On the other hand, cornu et al.  used the reaction route of alkylaminoborane and TCB to obtain polymers mainly connected by triatomic bridges -n-b-n-between rings. Cao Yimiao et al.  of Donghua University used methylamine to replace TCB to obtain trimethylamino cycloborazane. Then it is heated and polymerized with trimethylaminoborane to obtain a light yellow precursor. Its glass transition temperature is about 65 ℃, and the diameter of 45 can be obtained by drawing μ M primary fiber, which is cracked at high temperature in nitrogen and ammonia atmosphere to obtain gray brittle boron nitride fiber with a diameter of 20 μ m. The surface is smooth and dense without obvious defects.
Chen Mingwei, et al. [36, 37] of the Chinese Academy of Sciences synthesized a boron based polymer precursor through trimethylamino cycloborazine and trimethylamino borane in a similar way. The reaction principle is shown in Figure 5. The precursor is a transparent solid with a weight average molecular weight (MW) of 2536 and a softening point of 61 ℃. Melt spinning was heated in nitrogen atmosphere and solidified in ammonia atmosphere to obtain a diameter of 13 μ M, the fiber structure is compact, uniform, and the surface is smooth. BN fibers were obtained by heating to 1000 ℃ in ammonia atmosphere and then heating to 1600 ℃ in nitrogen atmosphere. The literature also points out that the heating rate has a great influence on the structure of BN fiber in the process of high temperature cracking. When the heating rate is high, hollow BN fibers will be formed. Researchers believe that during the curing process, the fiber surface is preferentially solidified in contact with NH3 to form a hard shell with high viscosity and maintain the morphology of the fiber. Due to the temperature gradient in the fiber diameter direction, the central viscosity of the fiber is lower than the surface viscosity, maintaining a certain fluidity. When heated, it shrinks outward along the diameter direction, forming a hollow pore structure. The lower the heating rate is, the smaller the pore size is. When it is lower than 0.1 ℃ /min, solid BN ceramic fibers can be obtained. The diameter of hollow BN fiber is 9.0 μ m. Aperture is 3.9 μ m. The carbon residue is low, the dielectric constant can reach 2.90, and the dielectric loss tangent is as low as 0.00064. It has excellent wave transmission performance and excellent oxidation resistance in the air at 950 ℃.
1.3 boron trichloride as monomer
Some researchers used organic amines to directly replace the Cl atom in BCl3 to synthesize a class of reactive amino borazine monomers. Different organic amines can synthesize monomers with different properties, heat crosslink into rings, and obtain polymer precursors after thermal polymerization. Ammonia treatment can increase the degree of crosslinking and reduce the carbon content at the same time .
Cornu et al.  used trimethylaminoborane as raw material to synthesize b-trimethylamino-n-trimethylcycloborazane monomer by heating, and then polymerized it to obtain the precursor. The reaction principle is shown in Figure 6. Its weight average molecular weight (MW) is about 900 and TG is 73 ℃. After melt spinning, the fibers are pyrolyzed in ammonia atmosphere to obtain hollow tubular BN fibers. The literature points out that this is due to the transamination reaction during pyrolysis. The reaction principle is shown in Figure 7. When the temperature is low, the surface of the precursor fiber contacts with ammonia to produce the monomer b-amino-n-methyl cycloborazane. During the heating process, b-amino-n-methyl cycloborazane polymerizes to form a thermosetting polymer. After the temperature rises, the interior of the fiber contacts with ammonia to undergo transamination reaction to generate b-amino-n-methyl cycloborazane monomer, which sublimates directly at this temperature and disappears, thus forming a hollow tubular structure. Researchers believe that the reason for the transamination reaction may be that the N atom on the b3n3 ring in the precursor is connected to the alkyl group.
Ye Li et al.  in China reacted with boron trichloride and hexamethyldisilazane (hmdz) as raw materials in a molar ratio of 1:2. The obtained monomer is further heated and polymerized to obtain polymer precursor. The reaction principle is shown in Figure 8. The molecular weight of the precursor obtained varies with the heating temperature. A soluble and fusible white solid was obtained at 160 ℃, with a number average molecular weight (MN) of 1264 and a weight average molecular weight (MW) of 4128. At 220 ℃, the soluble, non melting and softening precursor is obtained, with a number average molecular weight (n) of 2139 and a weight average molecular weight (MW) of
7 582。 This method has mild synthetic conditions, simple process and easy industrialization. The ceramic yield is 41.6% after high-temperature vitrification in nitrogen atmosphere. White boron nitride ceramics with low carbon content (0.14% (WT)) can be obtained by pyrolysis in ammonia atmosphere. The boron nitride ceramics with high crystallinity can be obtained by heating up to 1500 ℃.
Li Wenhua et al. [39-41] used boron trichloride and hexamethyldisilazane as raw materials in a similar way, and added excessive hmdz to replace the role of triethylamine precipitator to prepare the precursor silicon containing polyborazane in one step. The melting point is 108 ℃. The diameter of 20 ~ 25 can be obtained by heating to 190 ℃ in nitrogen atmosphere μ M organic fiber, which has dense structure, smooth surface, uniform diameter and flat section. HCl and BCl3 were successively introduced for desilication and non melting treatment, and BN ceramic fibers were obtained by heating to 1000 ℃ in ammonia. The silicon content of the fiber is very small, and the diameter is about 11 μ m. The section is flat and brittle, and its tensile strength is 0.45 GPa.
At present, BN fibers are prepared by two-step method with trichloro cycloborazane as monomer. Because the molecular structure of the precursor can be designed by using different nucleophiles, the processability of the precursor can be improved, and the physical properties of the final BN fibers can be affected. Therefore, using this method to study the preparation of BN fibers is a research hotspot at home and abroad. At this stage, the main focus of research is to control the polymerization process and the molecular structure of the precursor by adopting different routes and improving process conditions, so that the precursor has good processability and can be transformed into high-performance BN fiber. Therefore, the research on the new development of BN fiber precursors has the following directions: (1) the synthesis of new monomers and the design of polymer precursors, (2) air insensitive precursors, (3) controllable polymerization reaction, (4) the process conditions of synthesizing precursors are relatively harsh, the synthesis cost is high, and mass production cannot be achieved, which needs to be improved. (5) The pyrolysis process of primary fiber needs further study. (6) The mechanical properties of BN fiber need to be further improved.
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