ceramic fiber
The current level of development of technology makes it possible to obtain fibers from various substances and materials and thus provide the necessary set of physico-chemical characteristics for each specific application. Some types of fibers - polymer, glass, metal - have been successfully produced for a long time using proven technologies, being quite traditional materials in their fields. Others, such as carbon and ceramic, are of particular interest at the present stage of the development of chemical technology, since their use makes it possible to create materials of a new generation - light, strong, wear-resistant, for use at elevated temperatures and in aggressive environments.
Initially, in the early 1970s, oxide ceramic fibers were used as high-temperature heat-shielding materials, resistant up to 1600 °C, but not designed for any serious mechanical load. Ceramic fibers of small (no more than 10–20 μm) diameter have received a new round of development since the need arose to obtain reinforcing fibers for ceramic and metal composites with an application temperature above 500 °C. For the successful application of ceramic fibers in the creation of such innovative materials, in addition to chemical and thermal stability at elevated temperatures, a number of other requirements are imposed on them. The first of these is sufficient flexibility - in order to be able to manufacture blanks of various shapes and sizes for further molding of the composite. Sufficient flexibility, even for materials with a high modulus of elasticity, is provided by the small diameter of the fibers - the flexibility is inversely proportional to the fourth power of the fiber diameter. For example, to obtain a fiber of aluminum oxide or silicon carbide with an elastic modulus of 300 GPa, a diameter of 10 μm is required. Also, for greater manufacturability of the process of obtaining composites, the value of the minimum value of the relative elongation of the fiber before failure is regulated: it should not be lower than 1%. This entails a fiber strength requirement: the minimum tensile strength of a fiber with an elastic modulus of 200 GPa must be 2 GPa. To facilitate the created materials and structures, requirements are also imposed on the density of the fiber - it should not exceed 5 g / cm 3 . Long-term chemical and thermal stability and creep resistance at temperatures above 1100 °C are essential.
In chemical technology, fibers and fibrous materials play a truly enormous role. The current level of development of technology makes it possible to obtain fibers from various substances and materials and thus provide the necessary set of physico-chemical characteristics for each specific application. Some types of fibers - polymer, glass, metal - have been successfully produced for a long time using proven technologies, being quite traditional materials in their fields. Others, such as carbon and ceramic, are of particular interest at the present stage of the development of chemical technology, since their use makes it possible to create materials of a new generation - light, strong, wear-resistant, for use at elevated temperatures and in aggressive environments.
Ceramic fibers are all non-metallic fibers (oxide and non-oxide) with the exception of fibers obtained from glass melts. The boundary between glass and oxide ceramic fibers is not so easy to draw, since ceramic fibers obtained by sol-gel technology can be amorphous, and in this sense similar to glass fibers; on the other hand, methods for producing ceramic fibers have recently been developed, including the production of an oxide charge melt. The term "glass" should refer to fibers obtained from melts of a silicate composition; the main group of oxide "ceramic" fibers are aluminum oxide fibers, although there are other fibers from 4 5 high-temperature oxides. A conditional gradation between glass and ceramic fibers can also be carried out according to the temperature of their application: the first can only be used up to 1150 ° C (silica fiber), the second - at least up to 1400 ° C (in the case of SiC fibers in a non-oxidizing atmosphere) and 1600 ° C (for high-temperature oxide fibers based on Al2O3), and in some cases up to 2000 and 2500 ° C (fibers from ZrO 2).
Among refractory oxides: calcium, magnesium, aluminum, beryllium, zirconium, hafnium, thorium, etc., as well as many mixed oxides, industrial production of fibers has been established only for aluminum oxide. There are several reasons for this. The first is the wide distribution of raw materials: aluminum oxide and its derivatives are obtained from natural minerals bauxites, nephelines, kaolins, which gives a number of advantages over oxides of zirconium, hafnium, thorium, etc., the initial minerals for the production of which are inaccessible. The second is the high hardness of corundum (α-modification of aluminum oxide) - 9 on the Mohs scale - unlike, for example, CaO and MgO with a hardness of 5-6 on the Mohs scale. The third is technological features: industrial methods of spinning such high-temperature fibers, as a rule, include the production of oxide sols, which is not prone to oxides of group II elements. The fourth is the insufficient chemical stability of some refractory oxides. For example, calcium oxide reacts with water already at room temperature. The high chemical and thermal stability of aluminum oxide is due to its structure
In a single class, not only aluminum oxide fibers are distinguished, but also fibers with a high content of alumina. Such fibers can be classified according to several positions:
1. According to the degree of crystallinity: single crystals, polycrystalline. Amorphous ceramic fibers are not extremely rare.
2. By chemical composition. Allocate fibers from pure alumina (more than 99 wt% Al2O3); fibers containing 65 or more wt.% Al2O3 (separately, fibers containing the mullite phase 3Al2O3 ⋅ 2SiO2 , as well as a number of fibers of intermediate composition containing both mullite and aluminum oxide phases should be noted here). The composition of such fibers may additionally include oxides of zirconium, boron, alkali and alkaline earth metals.
3. By length: continuous and discrete (cut and staple).
4. According to the method of obtaining: direct and indirect methods are distinguished. In the first case, the fibers or their fibrous pre-ceramic precursors are obtained directly. These methods include sol-gel technologies and the production of an oxide from a melt (in the general case, a mixture of several oxides). Indirect methods include those methods that already finished fibers are used as starting materials: impregnation of industrially produced fibers with metal compounds and CVD technologies. CVD is used for coating, for example, W-fibers. The method can also be used directly to obtain fibers from the gas phase.
5. According to the fiber forming method: extrusion, blowing, mechanical methods (using centrifuges), electrospinning. Note that in practice there are also more complex methods for obtaining/spinning a fiber, including a combination of several of the above.
