Lakhasly

2.3.2.1 Chemical Vapor-Deposited (CVD) SiC Fibers CVD was the first approach used for the production of SiC fiber core-shell composite filaments.The production capacity reached an annual output of 12 kg. The dispersion coefficient of the tensile strength was less than 10%, and the continuous length was longer than 1000 m.

2.3.2.2 Pre-ceramic Polymer-Derived (PPD) SiC Fibers The precursor approach of transferring organic materials into inorganic materials by high-temperature treatment under oxygen-free atmospheres has been used since ancient times.The process includes four steps: (1) synthesis of pre-ceramic polymers (precursors); (2) melt spinning of polymers into green fibers; (3) curing of green fibers by oxidation or EB radiation, and (4) pyrolysis of the cured fibers under an inert atmosphere at high temperatures.Additionally, B-containing SiC fibers with improved mechanical properties can also be obtained by mixing a polycarbosilane precursor with polyborosilazane or polyborazine, or by curing polycarbosilane fibers in a B-containing atmosphere, such as BCl3.In 1987, Si3N4 fibers were prepared by Dow Corning by the synthesis of polymers from chlorosilane and hexamethyldisilazane (HMDS), melt spinning, curing in a chlorosilane atmosphere and pyrolysis at up to 1200 ?C under an inert atmosphere.Similar to the method to obtain carbon fibers wherein PAN or other organic fibers are carbonized in an inert atmosphere at high temperatures, this kind of precursor method has been applied to the commercial production of ceramic fibers [23].Ti and Zr only increase the thermal stability of SiC fibers to a limited extent, and more stable fibers are obtained upon the incorporation of Al and B. These elements act as sintering agents at higher temperatures.The Tyranno SA fibers by Ube Industries and the Sylramic fibers by Dow Corning have been produced by the sintering effect of Al and B near-stoichiometric SiC fibers, as listed in Table 2.25.Three typical Si3N4 fibers are available from the Dow Corning Corporation in the USA, Toa Nenryo Kogyo K. K. (TNK) in Japan and Domaine University in France, and these represent three different preparation technologies.(4) Functional SiC Fibers Based on their high strength, high modulus, low coefficient of thermal expansion and adjustable electrical resistivity, SiC fibers are not only good reinforcements for structural composites but are also good high-temperature radar-absorbing reinforcements for functional composites.Fortunately, precursor varieties, processing parameters and ultimate microstructures (cross section of the fibers) can all be used to achieve this target, and radar-absorbing fibers with the best absorption capacities in the range of 10- 12 GHz can be obtained [39-42].The tensile strength of the fibers was 2.6 GPa, and the continuous length reached 900 m. The radio-frequency heating method was then successfully applied to produce CVD SiC (W core) fibers.Newer silicon-based ceramic fibers have also produced such as M-containing SiC fibers (M = Ti, Zr, Al, etc.), near-stoichiometric SiC fibers, silicon nitride (Si3N4) and Si-B-C-N fibers [23, 25, 30-41].These fibers were applied, respectively, to reinforce polymers, aluminum, titanium, intermetallic compounds and ceramic matrixes.In addition, PPD SiC fibers are very good heat-resistant materials and can be used as insulation materials, high-temperature conveying belts, melt filters, etc.Polycarbosilane-derived Nicalon fibers (NL-200, for example) are not pure SiC fibers as they contain oxygen (14.0 wt%) and a trace of hydrogen (0.15 wt%) in addition to silicon (55.5 wt%) and carbon (28.4 wt%).The high-temperature mechanical properties of Nicalon fibers are limited because of the unstable SiCxOy phase, which will undergo decomposition upon an increase in the grain size of b-SiC at temperatures higher than 1200 ?C.(2) M-Containing SiC Fibers (M = Ti, Zr, Al, B) Another effective method to improve Nicalon fibers is the introduction of metal or other nonmetallic elements such as Ti, Zr, Al and B [32-39].Upon heating to 1400 ?C for 1 h under argon, their retained tensile strengths are both over 95% of the original value, confirming that their thermal stabilities are better than that of the Nicalon NL201 fibers.The precursor can be cross-linked by heating giving an infusible but still soluble fiber, and therefore, it is suitable for the preparation of high-purity Si3N4 fibers without other elements.Because of the limited thermal stability of Si-C-N fibers, B was introduced and an amorphous Si-B-N-C fiber, SiBN3C, is currently being developed by Bayer HG in Germany.China entered this field in the early 1980s, and a variety of SiC fibers have been studied including carbon-rich SiC fibers, magnetic particle-containing SiC fibers and non-circular SiC fibers.The improvement in mechanical properties upon titanium introduction is due to a generation of TiC microcrystals preventing the growth of b-SiC crystals.Another type of Nicalon fiber is the high-volume-resistivity (HVR) type such as NL-400, which has an electrical resistance of 106-107 X_cm, and can be used as an excellent radar transmission fiber.Continuous SiC (W core) fibers with surface protection coatings were successfully manufactured with properties close to the similar US and UK products in the 1990s (see Table 2.27).Compared with CVD SiC fibers, the biggest advantage of PPD SiC fibers is their much smaller diameter, which allows easy weaving into a variety of fabrics.After several years of research, important progress was made such as the synthesis of polycarbosilane at normal pressure, multi-spinneret melt spinning, continuous curing and continuous pyrolysis.The tensile strength of the continuous fibers ranged from 2.6 to 3.0 GPa with Young's moduli of 150-190 GPa and diameters of 12-15 lm, which are close to those of the Nicalon fibers.Ti-, Zr- andAl-containing SiC fibers have been developed by Ube Industries in Japan.Compared with the Nicalon fibers, the obvious advantages of Tyranno fibers are their higher thermal stability and good compatibility with aluminum and aluminum alloys.China has also developed Al- and B-doped SiC fibers from polyaluminocarbosilane or a hybrid precursor of polyborazine and polycarbosilane [34, 35].They are mainly used in metal matrix composites (MMC), ceramic matrix composites (CMC) and heatproof composite materials.These Si3N4 fibers are good reinforcing candidates for CMCs and MMCs because of their high thermal stabilities and oxidation resistance.Domaine University used polycarbosilazane as the precursor for Si3N4-SiC fibers, and this was synthesized from chlorosilanes by ammonolysis and polymerization.Low oxygen content Si3N4 fibers were also obtained by electron beam irradiation, but their mechanical properties need to be improved.For example, the electrical resistance of Nicalon fibers of low-volume-resistivity (LVR) type, NL-500, is 0.5-5.0 X_cm.Systematic studies were carried out on non-circular fibers by changing the shapes of the spinnerets upon melt-spinning polycarbosilanes [38].The results showed that at the same equivalent diameters, the tensile strength of the fibers with a trilobal cross section is about 30% higher than that with a circular cross section, and the rate of tensile strength reduction upon increasing the diameter is also lower.The electromagnetic parameters of trilobal SiC fibers measured using a rectangular waveguide approach in the X-band are listed in Table 2.30.The electromagnetic parameters of the trilobal SiC fibers are similar to those of the circular fibers (NL202) at lower pyrolysis temperatures.In 1961, Gareis and coworkers applied for a patent using ultra-fine W silk as the deposition support to produce SiC (W core) fibers [28].Meanwhile, detailed studies on the reaction mechanism revealed characteristics such as microstructure and optimal parameters.Nippon Carbon Co. first realized the industrial production of a series of continuous SiC fibers under the trademark Nicalon.(1) Continuous SiC fibers In the 1970s, Professor Yajima first obtained SiC fibers from a silicon-based polymer, polycarbosilane.Based on this modification, oxygen-free Hi-Nicalon and Hi-Nicalon type S fibers have been produced by Nippon Carbon, and these fibers can withstand temperatures up to 1500-2000 ?C.China started to synthesize polycarbosilane from polysilane according to a modified Yajiam route at normal pressure using domestic raw materials.China also developed Ti-containing SiC fibers and obtained continuous fibers longer than 300 m with a filament count of 400-600 [33].The composition, microstructure and properties of the Si3N4 fibers differ greatly depending on the polymers.If their green fibers are cured by c-ray irradiation, oxygen-free Si-C-N fibers can be obtained with a tensile strength and Young's modulus of 2.4 and 214 GPa, respectively.However, when the pyrolysis temperature was increased to 1100 ?C or 1250 ?C, the imaginary part of the permittivity (e?) of the trilobal SiC fibers is about 30-60 times that of circular fibers.Recently, the US company Textron (formerly AVCO Corporation) was allowed to produce a series of SCS-2, SCS-6 and SCS-8 SiC (C core) fibers.Research has been carried out in China on mercury electrode-heated CVD SiC (W core) fibers as early as 1975 [29].Fine-diameter continuous SiC fibers are finally obtained from polycarbosilane precursors.Therefore, Nicalon fibers are not good enough to be used as heat-resistant materials or as advanced composite reinforcements.One effective method is to reduce oxygen incorporation using oxygen-free approaches such as electron beam curing [31, 32].Furthermore, they have a low coefficient of thermal expansion, low thermal conductivity, good thermal shock resistance, good oxidation resistance and good insulation.The processing of Si3N4 fibers is similar to that of SiC fibers in terms of synthesis, spinning, curing and pyrolysis.Their pre-ceramic polymers are polysilazanes or polycarbosilazanes, which can be synthesized in various strategies.This fiber has high room-temperature strength and stiffness and is reported to have remarkable strength retention and creep resistance at elevated temperatures.(1) Non-circular SiC fibers A reason to change the cross section of SiC fibers from circular to non-circular can be explained using carbon fibers, which are radar reflection fibers with an electric resistance of about 10-2 X_cm.In 1972, US company AVCO Corporation produced large-diameter C-wire, and as a result, SiC (C core) fibers were produced with better performance and lower cost.Subsequently, from 1981 to 1984, SiC (C core) fibers were successfully commercialized by AVCO.The successful development of SiC fibers by the PPD method resulted in a large amount of interest from material scientists.Recently, plenty of research has been carried out on Nicalon fibers and their composites resulting in an understanding of their advantages and disadvantages.These elements are in the b-SiC (1-5 nm), SiCxOy and free carbon forms, respectively.However, this precursor is unstable in air, and the SiC fibers obtained had low tensile strength.The fiber diameter range is 14-16 lm, the tensile strength range is 2.20-2.80 GPa and the Young's moduli range is 160-180 GPa.Their tensile strengths are 2.8 and 2.2 GPa, respectively, with diameters of 12 lm and 13 lm, respectively.(3) Silicon Nitride (Si3N4) and Si-B-C-N Fibers As another type of important Si-based ceramic fibers, silicon nitride (Si3N4) fibers also have excellent mechanical properties [25].They synthesized a hydropolysilazane precursor via ammonalysis of dichlorosilane.The precursor only contains Si, N and H, resulting in plenty of Si-H bonds and N-H bonds and thus a very reactive material.Their diameter, tensile strength and Young's modulus are 20 lm, 1.85 and 186 GPa, respectively.It produced a series of fibers referred to as SM1040, SM1140 and SM1240 with different surface coatings.This is the first ceramic fiber obtained using polymer techniques.Subsequently, Nippon Carbon procured the patent and started scale-up production.A series of fibers were then commercialized and trademarked as Nicalon, as listed in Table 2.28.They are thus more suitable for reinforcing aluminum matrixes.In addition, the creep resistance of the Al-containing SiC fibers is better than that of the Nicalon fibers.There are thus two strategies for the preparation of target Si3N4 fibers, pure Si3N4 fibers and Si3N4-SiC fibers.Their diameter is 10- 15 lm, their tensile strength is 3.1 GPa and their Young's modulus is 260 GPa.China has also carried out a series of similar studies and obtained Si-C-N-O fibers from chlorosilane.Pure SiC is a semiconducting material with an electrical resistance lies in the range of 104-106 X_cm.The properties of Nicalon serial fibers are listed in Table 2.28.These SiC fibers have been commercialized and have the trade name Tyranno.They can withstand temperatures up to 2000 ?C just like the Hi-Nicalon type S fibers.TNK began research into Si3N4 fibers slightly later than Dow Corning.Because Si3N4 and SiC coexist in the Si-C-N-O fibers, they are expected to have new features.After 1600 ?C treatment, their tensile strength and Young's modulus are still as high at 2.1 and 220 GPa, respectively.For use as good radar absorbents, their electrical resistance should be in the range of 101-103 X_cm.Therefore, measures are required to adjust their electrical resistance.They are all reinforcements with functional properties, as listed in Table 2.28.British company BP bought the original German technology for the production of SiC (W core) fibers.This lay the foundation for further enhancements of fiber performance and a reduction in production costs.Recently, a kind of carbon-free Si-B-N fiber has been developed in China [36].It can then be easily used as reinforcements in complicated composites.In addition, the fiber surface is oxygen-rich in the form of SiO2.However, more oxygen was also introduced, which has negative influence on their thermal stabilities.The ultimate fibers are stoichiometric Si3N4.Their properties are listed in Table 2.29.They have good radar-absorbing properties.They can also absorb microwaves if the shape and size of the cross section are changed.

2.3.2.1 Chemical Vapor-Deposited (CVD) SiC Fibers
CVD was the first approach used for the production of SiC fiber core–shell composite
filaments. In 1961, Gareis and coworkers applied for a patent using ultra-fine
W silk as the deposition support to produce SiC (W core) fibers [28]. In 1972, US
company AVCO Corporation produced large-diameter C-wire, and as a result, SiC
(C core) fibers were produced with better performance and lower cost.
Subsequently, from 1981 to 1984, SiC (C core) fibers were successfully commercialized
by AVCO. Recently, the US company Textron (formerly AVCO
Corporation) was allowed to produce a series of SCS-2, SCS-6 and SCS-8 SiC (C
core) fibers.
British company BP bought the original German technology for the production
of SiC (W core) fibers. It produced a series of fibers referred to as SM1040,
SM1140 and SM1240 with different surface coatings. These fibers were applied,
respectively, to reinforce polymers, aluminum, titanium, intermetallic compounds
and ceramic matrixes.
Research has been carried out in China on mercury electrode-heated CVD SiC
(W core) fibers as early as 1975 [29]. The tensile strength of the fibers was 2.6 GPa,
and the continuous length reached 900 m. The radio-frequency heating method was
then successfully applied to produce CVD SiC (W core) fibers. Meanwhile, detailed
studies on the reaction mechanism revealed characteristics such as microstructure
and optimal parameters. Continuous SiC (W core) fibers with surface protection
coatings were successfully manufactured with properties close to the similar US and
UK products in the 1990s (see Table 2.27). The production capacity reached an
annual output of 12 kg. The dispersion coefficient of the tensile strength was less
than 10%, and the continuous length was longer than 1000 m.

2.3.2.2 Pre-ceramic Polymer-Derived (PPD) SiC Fibers
The precursor approach of transferring organic materials into inorganic materials by
high-temperature treatment under oxygen-free atmospheres has been used since
ancient times. Similar to the method to obtain carbon fibers wherein PAN or other
organic fibers are carbonized in an inert atmosphere at high temperatures, this kind
of precursor method has been applied to the commercial production of ceramic
fibers [23].
The process includes four steps: ① synthesis of pre-ceramic polymers (precursors);
② melt spinning of polymers into green fibers; ③ curing of green fibers
by oxidation or EB radiation, and ④ pyrolysis of the cured fibers under an inert
atmosphere at high temperatures. Fine-diameter continuous SiC fibers are finally
obtained from polycarbosilane precursors. Nippon Carbon Co. first realized the
industrial production of a series of continuous SiC fibers under the trademark
Nicalon.
Compared with CVD SiC fibers, the biggest advantage of PPD SiC fibers is their
much smaller diameter, which allows easy weaving into a variety of fabrics. It can
then be easily used as reinforcements in complicated composites. In addition, PPD
SiC fibers are very good heat-resistant materials and can be used as insulation
materials, high-temperature conveying belts, melt filters, etc. The properties of
Nicalon serial fibers are listed in Table 2.28.
The successful development of SiC fibers by the PPD method resulted in a large
amount of interest from material scientists. Recently, plenty of research has been
carried out on Nicalon fibers and their composites resulting in an understanding of
their advantages and disadvantages. This lay the foundation for further enhancements
of fiber performance and a reduction in production costs. Newer
silicon-based ceramic fibers have also produced such as M-containing SiC fibers
(M = Ti, Zr, Al, etc.), near-stoichiometric SiC fibers, silicon nitride (Si3N4) and
Si–B–C–N fibers [23, 25, 30–41]. China entered this field in the early 1980s, and a
variety of SiC fibers have been studied including carbon-rich SiC fibers, magnetic
particle-containing SiC fibers and non-circular SiC fibers.
(1) Continuous SiC fibers
In the 1970s, Professor Yajima first obtained SiC fibers from a silicon-based
polymer, polycarbosilane. This is the first ceramic fiber obtained using polymer
techniques. Subsequently, Nippon Carbon procured the patent and started scale-up
production. A series of fibers were then commercialized and trademarked as
Nicalon, as listed in Table 2.28.
Polycarbosilane-derived Nicalon fibers (NL-200, for example) are not pure SiC
fibers as they contain oxygen (14.0 wt%) and a trace of hydrogen (0.15 wt%) in
addition to silicon (55.5 wt%) and carbon (28.4 wt%). These elements are in the
b-SiC (1–5 nm), SiCxOy and free carbon forms, respectively. In addition, the fiber
surface is oxygen-rich in the form of SiO2.
The high-temperature mechanical properties of Nicalon fibers are limited
because of the unstable SiCxOy phase, which will undergo decomposition upon an
increase in the grain size of b-SiC at temperatures higher than 1200 °C. Therefore,
Nicalon fibers are not good enough to be used as heat-resistant materials or as
advanced composite reinforcements.
One effective method is to reduce oxygen incorporation using oxygen-free
approaches such as electron beam curing [31, 32]. Based on this modification,
oxygen-free Hi-Nicalon and Hi-Nicalon type S fibers have been produced by
Nippon Carbon, and these fibers can withstand temperatures up to 1500–2000 °C.
China started to synthesize polycarbosilane from polysilane according to a
modified Yajiam route at normal pressure using domestic raw materials. However,
this precursor is unstable in air, and the SiC fibers obtained had low tensile strength.
After several years of research, important progress was made such as the synthesis
of polycarbosilane at normal pressure, multi-spinneret melt spinning, continuous
curing and continuous pyrolysis. The tensile strength of the continuous fibers
ranged from 2.6 to 3.0 GPa with Young’s moduli of 150–190 GPa and diameters of
12–15 lm, which are close to those of the Nicalon fibers.

(2) M-Containing SiC Fibers (M = Ti, Zr, Al, B)
Another effective method to improve Nicalon fibers is the introduction of metal or
other nonmetallic elements such as Ti, Zr, Al and B [32–39]. Ti-, Zr- andAl-containing SiC fibers have been developed by Ube Industries in Japan. These
SiC fibers have been commercialized and have the trade name Tyranno. Compared
with the Nicalon fibers, the obvious advantages of Tyranno fibers are their higher
thermal stability and good compatibility with aluminum and aluminum alloys. They
are thus more suitable for reinforcing aluminum matrixes.
Ti and Zr only increase the thermal stability of SiC fibers to a limited extent, and
more stable fibers are obtained upon the incorporation of Al and B. These elements
act as sintering agents at higher temperatures. The Tyranno SA fibers by Ube
Industries and the Sylramic fibers by Dow Corning have been produced by the
sintering effect of Al and B near-stoichiometric SiC fibers, as listed in Table 2.25.
They can withstand temperatures up to 2000 °C just like the Hi-Nicalon type S
fibers.
China also developed Ti-containing SiC fibers and obtained continuous fibers
longer than 300 m with a filament count of 400–600 [33]. The fiber diameter range
is 14–16 lm, the tensile strength range is 2.20–2.80 GPa and the Young’s moduli
range is 160–180 GPa. The improvement in mechanical properties upon titanium
introduction is due to a generation of TiC microcrystals preventing the growth of
b-SiC crystals. However, more oxygen was also introduced, which has negative
influence on their thermal stabilities.
China has also developed Al- and B-doped SiC fibers from polyaluminocarbosilane
or a hybrid precursor of polyborazine and polycarbosilane [34, 35]. Their
tensile strengths are 2.8 and 2.2 GPa, respectively, with diameters of 12 lm and
13 lm, respectively. Upon heating to 1400 °C for 1 h under argon, their retained
tensile strengths are both over 95% of the original value, confirming that their
thermal stabilities are better than that of the Nicalon NL201 fibers. In addition, the
creep resistance of the Al-containing SiC fibers is better than that of the Nicalon
fibers. Additionally, B-containing SiC fibers with improved mechanical properties
can also be obtained by mixing a polycarbosilane precursor with polyborosilazane
or polyborazine, or by curing polycarbosilane fibers in a B-containing atmosphere,
such as BCl3.
(3) Silicon Nitride (Si3N4) and Si–B–C–N Fibers
As another type of important Si-based ceramic fibers, silicon nitride (Si3N4) fibers
also have excellent mechanical properties [25]. Furthermore, they have a low
coefficient of thermal expansion, low thermal conductivity, good thermal shock
resistance, good oxidation resistance and good insulation. They are mainly used in
metal matrix composites (MMC), ceramic matrix composites (CMC) and heatproof
composite materials.
The processing of Si3N4 fibers is similar to that of SiC fibers in terms of synthesis,
spinning, curing and pyrolysis. Their pre-ceramic polymers are polysilazanes
or polycarbosilazanes, which can be synthesized in various strategies. The composition,
microstructure and properties of the Si3N4 fibers differ greatly depending
on the polymers. There are thus two strategies for the preparation of target Si3N4
fibers, pure Si3N4 fibers and Si3N4–SiC fibers. Three typical Si3N4 fibers are available from the Dow Corning Corporation in
the USA, Toa Nenryo Kogyo K. K. (TNK) in Japan and Domaine University in
France, and these represent three different preparation technologies.
In 1987, Si3N4 fibers were prepared by Dow Corning by the synthesis of
polymers from chlorosilane and hexamethyldisilazane (HMDS), melt spinning,
curing in a chlorosilane atmosphere and pyrolysis at up to 1200 °C under an inert
atmosphere. The ultimate fibers are stoichiometric Si3N4. Their diameter is 10–
15 lm, their tensile strength is 3.1 GPa and their Young’s modulus is 260 GPa.
TNK began research into Si3N4 fibers slightly later than Dow Corning. They
synthesized a hydropolysilazane precursor via ammonalysis of dichlorosilane. The
precursor only contains Si, N and H, resulting in plenty of Si–H bonds and N–H
bonds and thus a very reactive material. The precursor can be cross-linked by
heating giving an infusible but still soluble fiber, and therefore, it is suitable for the
preparation of high-purity Si3N4 fibers without other elements. These Si3N4 fibers
are good reinforcing candidates for CMCs and MMCs because of their high thermal
stabilities and oxidation resistance. Their properties are listed in Table 2.29.
Domaine University used polycarbosilazane as the precursor for Si3N4–SiC
fibers, and this was synthesized from chlorosilanes by ammonolysis and polymerization.
Because Si3N4 and SiC coexist in the Si–C–N–O fibers, they are
expected to have new features. Their diameter, tensile strength and Young’s
modulus are 20 lm, 1.85 and 186 GPa, respectively. If their green fibers are cured
by c-ray irradiation, oxygen-free Si–C–N fibers can be obtained with a tensile
strength and Young’s modulus of 2.4 and 214 GPa, respectively. After 1600 °C
treatment, their tensile strength and Young’s modulus are still as high at 2.1 and
220 GPa, respectively.
Because of the limited thermal stability of Si–C–N fibers, B was introduced and
an amorphous Si–B–N–C fiber, SiBN3C, is currently being developed by Bayer HG
in Germany. This fiber has high room-temperature strength and stiffness and is
reported to have remarkable strength retention and creep resistance at elevated
temperatures.
China has also carried out a series of similar studies and obtained Si–C–N–O
fibers from chlorosilane. Low oxygen content Si3N4 fibers were also obtained by
electron beam irradiation, but their mechanical properties need to be improved.
Recently, a kind of carbon-free Si–B–N fiber has been developed in China [36].
(4) Functional SiC Fibers
Based on their high strength, high modulus, low coefficient of thermal expansion
and adjustable electrical resistivity, SiC fibers are not only good reinforcements for
structural composites but are also good high-temperature radar-absorbing reinforcements
for functional composites.
For use as good radar absorbents, their electrical resistance should be in the
range of 101–103 X_cm. Pure SiC is a semiconducting material with an electrical
resistance lies in the range of 104–106 X_cm. Therefore, measures are required to
adjust their electrical resistance. Fortunately, precursor varieties, processing parameters and ultimate
microstructures (cross section of the fibers) can all be used to achieve this target,
and radar-absorbing fibers with the best absorption capacities in the range of 10–
12 GHz can be obtained [39–42].
For example, the electrical resistance of Nicalon fibers of low-volume-resistivity
(LVR) type, NL-500, is 0.5–5.0 X_cm. They have good radar-absorbing properties.
Another type of Nicalon fiber is the high-volume-resistivity (HVR) type such as
NL-400, which has an electrical resistance of 106–107 X_cm, and can be used as an
excellent radar transmission fiber. They are all reinforcements with functional
properties, as listed in Table 2.28.
① Non-circular SiC fibers
A reason to change the cross section of SiC fibers from circular to non-circular can
be explained using carbon fibers, which are radar reflection fibers with an electric
resistance of about 10−2 X_cm. They can also absorb microwaves if the shape and
size of the cross section are changed.
Systematic studies were carried out on non-circular fibers by changing the
shapes of the spinnerets upon melt-spinning polycarbosilanes [38]. The results
showed that at the same equivalent diameters, the tensile strength of the fibers with
a trilobal cross section is about 30% higher than that with a circular cross section,
and the rate of tensile strength reduction upon increasing the diameter is also lower.
The electromagnetic parameters of trilobal SiC fibers measured using a rectangular
waveguide approach in the X-band are listed in Table 2.30. The electromagnetic
parameters of the trilobal SiC fibers are similar to those of the circular
fibers (NL202) at lower pyrolysis temperatures. However, when the pyrolysis
temperature was increased to 1100 °C or 1250 °C, the imaginary part of the permittivity
(e″) of the trilobal SiC fibers is about 30–60 times that of circular fibers.
A higher e” is beneficial because it implies a better ability to transfer microwave
energy to heat. The reflection curve of a composite with two-orthogonal-layered
trilobal SiC fibers is shown in Fig. 2.2 [38]. The size of the composite is
80 mm _ 180 mm _ 4 mm.
The reflection is lower than −10 dB in the range of 11.6–18.0 GHz or lower
than −15 dB in the range of 13.9–18.0 GHz. The lowest reflection of –19.8 dB is
achieved at 17.2 GHz. ② Metal particle-containing SiC fibers A kind of metal particle-containing SiC
fiber was produced by simply mixing metal particles with precursors followed
by subsequent melt spinning, curing and pyrolysis [39–41]. Changing the
metal content can change the electrical resistance and the electromagnetic
parameters of the target SiC fibers. The metal particles studied were ultra-fine
metal powders of Fe, Co, Ni and Ti with 30–50 nm diameters. The electrical
resistance of the SiC fibers can be adjusted continuously over a wide range, as
shown in Figs. 2.3 and 2.4 and Table 2.31. The electrical resistance decreases
with increasing metal content, but the tensile strength decreases too.
When nano-ferrous powder was added to polycarbosilane, the electrical resistivity
of the obtained SiC fibers decreased sharply. For example, 1–5%(wt) of the
nano-ferrous powder effectively reduced the electrical resistance to 100–103 X_cm.
The fibers still retained a relatively high tensile strength, as listed in Table 2.31.
Figure 2.5 shows the reflection curve of an epoxy matrix composite reinforced with
Fe-containing SiC fibers with a thickness of 6 mm. The reflections are all less than −14 dB in the range of 8–12.4 GHz with the
lowest achieved being −25.1 dB. Additionally, the reflection bandwidth at less than
−20 dB is about 2.2 GHz, indicating a much improved microwave-absorbing
property with the addition of magnetic ferrous powder.
Other than Fe, Ti and Zr can also be introduced to the body of SiC fibers to
lower their electrical resistance while retaining their high tensile strength. UbeIndustries has developed and commercialized a series of Tyranno fibers for different
applications, and their properties are listed in Table 2.32.
Titanium can be introduced by reacting polycarbosilane with titanium alkoxide,
and the properties of the thus-obtained SiC fibers are listed in Table 2.33. It was
found that the elemental composition and microstructure are closely related to the
electrical resistance. With an increase in the titanium content, the resistance
decreased and a relatively high strength was retained.
Carbon is conductive and can be introduced to the SiC fibers to change their
electrical resistance. A kind of SiC–C fiber was prepared from a blended precursor
consisting of polycarbosilane and pitch. Because of an easy phase detachment, the
blended precursor is spun with difficulty into fine fibers. Therefore, even the
electrical resistance is adjustable in the range of 100–104 X_cm, and the tensile
strength is relatively low at about 1.0–1.2 GPa. They cannot be used as reinforcements
for structural radar-absorbing composites.
An improved method consists of the synthesis of carbon-rich polycarbosilane by
the co-thermolysis of polysilane and polyvinylchloride (PVC). When PVC is
pyrolyzed, plenty of double bonds are created, which can be grafted onto the main
chain of Si–C bonds by a hydrosilation reaction. Therefore, carbon was dispersed
into the precursor at the molecular level and the spinning property improved
greatly. The properties of the SiC–C fibers are listed in Table 2.34.
A substantial reduction in the electrical resistance is apparent. At a PVC content
of 5 wt% the electrical resistance is low enough to meet application requirements.
Additionally, the tensile strength is also slightly higher than for those without PVC
addition.
A different method exists for SiC–C fiber production and comprises increasing
the silicon content of the C fibers by the infiltration of carbon precursor fibers with
polycarbosilane. For example, rayon filament or natural cellulose fiber was infiltrated
with polycarbosilane solution, pre-oxidized and heat-treated at high temperature,
and a kind of SiC–C composite fiber was obtained [22]. The carbon yield
of the composite fiber was improved by up to 35–38%(wt) by the promotion of
pyrolysis with flame retardants consisting of multi-amines and surfactants.
Furthermore, the electrical resistance of the fibers can be adjusted by the concentration
of polycarbosilane solution. The tensile strength of the fiber was as high as
1.8 GPa. Figure 2.6 shows the reflection curve of this composite fiber, indicating
wideband microwave-absorbing properties.

2.3.3 Boron Nitride (BN) Fibers
As a variety of inorganic heat-resistant fiber, BN fibers are white, flexible polycrystalline
fibers. According to the manufacturing method and their microstructure,
they are usually divided into composite fibers and pure fibers [43, 44]. The former is prepared by CVD using borane, ammonia and boron trichloride as
gas vapors, and a hot W wire as the deposition support and the core of the composite
fiber. The latter usually comes from melt-spun B2O3 fibers after treatment
with NH3 to give unstable boron amine at low temperatures and heat resistance
polycrystalline BN at 1800 °C.
BN fibers have superior thermal insulation and high-temperature stability,
excellent electrical insulation and good dielectric properties; in addition, they have
good resistance to radiation, infrared rays and chemical corrosion. When used as
reinforcements for ceramic matrix composites, they can increase the toughness and
thermal shock resistance. They have been used to fabricate microwave window
components, separation rings in continuous casting technology and cell membranes
in communication satellites.
For example, BN fiber-reinforced quartz has been used as a missile antenna
window component and meets all the requirements of the space environment. BN
fiber-reinforced Si3N4 can survive the erosion of carbon steel and stainless steel at
1600 °C, indicating that the composite can withstand the thermal shock generated
by a tremendous temperature difference.
BN fibers can withstand long-term erosion by a 40 wt% KOH solution and as
cell membranes of alkaline batteries and high-energy batteries, they are corrosion
resistant, Ag2O migration resistant and stable at high temperatures with the ability
to retain the electrolyte. Therefore, BN fibers are a good cell membrane material.
Furthermore, BN fibers are ideal lubrication materials because of their good
high-temperature lubrication property, which comes from their structure similarity
with graphite. BN fibers can be used as protective clothing materials because they
have the capacity to absorb neutrons with resistance toward ultraviolet and cosmic
rays. Based on their excellent chemical stabilities, BN fibers can also be used in the
form of paper and carpets as chemical filters and gas filters.
The USA was the first country to produce BN fibers. The former Soviet Union
and Japan also carried out systematic studies into BN fibers. The properties of
typical BN fibers are listed in Table 2.35.
In 1966, the Emery Co. in the USA was the first to produce BN fibers and they
produced high-strength, high-modulus BN fibers in 1978. At the same time, various
BN fiber products such as paper, felts and boards were developed. In 1976, research
was carried out in China into reacting B2O3 with ammonia, and the relationship
between microstructure and performance was systematically studied. In 1993, BN
fibers with similar performance were obtained, as listed in Table 2.35.
Because of the complexity of the solid-gas reaction, the obtained BN fibers were
hardly homogenous [43]. In the 1990s, research focused on the manufacture of BN
fibers from polymer precursors to overcome the shortcomings of the nitridation
process [44]. Currently, the tensile strength of polymer-derived BN fibers is up to
1.5 GPa.
2.3.4 Boron Fibers
Boron fibers are important reinforcements for advanced composites. They are
produced by CVD by depositing B on W or C fibers in the form of a continuous
monofilament with an outer diameter of 100–200 lm.
The commonly used W wire has a diameter of 3.5–50 lm. At reaction temperatures
of 1120–1200 °C, more infiltration of B into W is found. Therefore, the
composition of the core changes from W to a variety of tungsten diborides such as
WB, W2B5 and WB4. As a result, only a small amount of boron is deposited into
the core. However, when the temperature is increased to 1200–1300 °C, the
deposition rate of B increases and the target B fibers can be obtained.
During the course of deposition, the core has a pressing stress and the initial
deposition layer has a drawing stress, and therefore, radial cracks form in the B
fibers. To avoid the propagation of cracks and any unexpected interface reactions, a
coating process is usually carried out in addition to the CVD process. Therefore, a
coating of boron carbide (B4C) is applied using a mixed gas consisting of BCl3,
CH4 and H2. The thickness of the coating is generally 3 lm. A commercialized B
fiber with the trademark BoSiC is obtained when the coating is SiC.
The most promising advantages of boron fibers are their mechanical properties
(tensile strength and Young’s modulus are 3.5 and 400 GPa, respectively) and low
density (2.5 g/cm3). The processing maturity and reasonable price are essential
reasons for their development. In addition, B fibers have good bending strength,
and their corresponding compressive strength is very high at 6.9 GPa, which is
twice their tensile strength. Boron fibers can survive at 500 °C in air for 1 h with no
obvious change in tensile strength. However, at temperatures exceeding 500 °C,
their tensile strength decreases significantly.
Boron fibers are mainly used in the aviation and aerospace industries with
special applications in MMC and PMC composites that have specific demands
regarding weight and stiffness. Boron fiber-reinforced aluminum composites are
one successful example. Boron fiber-reinforced epoxy composites have also been
used to repair airplane metal bodies and to fabricate sports and entertainment items
such as golf clubs and skis.
The USA was the earliest and most important country regarding the research and
development of boron fibers. In the mid-1960s, AVCO produced W core and C
core boron fibers using hydrogen and boron trichloride. The diameters of the
continuous fibers were 100–200 lm. Textron Inc. then produced a high-strength,
high-modulus and low-density boron fiber. In July 1985, the Japanese vacuum
metallurgy company developed the world’s highest tensile strength boron fiber at
5.2 GPa and established a pilot plant.
The former Soviet Union and France also carried out research and made
extensive progress. China began these studies in the early 1970s, and five pilot lines
have been completed. The performances of the fibers are close to those of other
countries. The properties of typical boron fibers are listed in Table 2.36. Their
performance is closely related to their diameters.