Liquid polymer infiltration

A further technological advantage of applying the polymer route for composite matrices is that no mechanical damage of the reinforcing fibers is caused. The scheme in Fig. 13 demonstrates the steps for making composites by the liquid polymer infiltration process [250, 251]. 

Fig. 13. Liquid polymer infiltration route for fabrication of SiC composite structures fi-om Si-precursors [fi om 251]...

LPI-liquid polymer infiltration CVI-chemical vapor infiltration... 

Table 7 contains a comparison of mechanical properties achieved for the three unconventional routes described to make SiC, composite matrices by LPI (liquid polymer infiltration), CVI (chemical vapor infiltration) and Si-infiltration [277]. 

Fabrication by Liquid Polymer Infiltration (LPI) In the first step of the LPI process, a carbon-fiber preform is infiltrated with resin (e.g. polycarbosilane), to bind the fibers together. Then the polymer is pyrolized to form SiC. These process steps are repeated a number of times until the pores are narrow enough that further... 

Liquid Polymer Infiltration (LPI) or Polymer Infiltration and Pyrolysis (PIP)... 

FIGURE 11. The technique for making C/SiC components by liquid polymer infiltration (LPI)... 

SiC- Sic and SiC-C (Continuous Fiber-Reinforced SiC Matrix Composites) Three different processes are commonly used to manufacture carbon fiber-reinforced SiC materials (i) chemical vapor infiltration (CVI) [340] (ii) liquid polymer infiltration (LPI also termed polymer infiltration and pyrolysis, PIP) [341]) and (iii) melt infiltration or liquid silicon infiltration (MI/LSI) [342]. 

Liquid-phase infiltration of preforms has emerged as an extremely useful method for the processing of composite materials. This process involves the use of low-viscosity liquids such as sols, metal- or polymer-melts. Using this infiltration process, it is possible to design new materials with unique microstructures (e.g. graded, multiphase, microporous) and unique thermomechanical properties (graded functions, designed residual strains and thermal shock). 

Ozaki, M., Shimoda, Y, Kasano, M. et al.. Electric field tuning of the stop band in a liquid-crystal-infiltrated polymer inverse opal, Adv. Mater., 14, 514, 2002. 

Silicon carbide (SiC) matrix composites have been fabricated by chemical vapor infiltration (CVl), polymer impregnation and pyrolysis (PIP), and reaction sintering (RS). The RS process can be recognized as an attractive technique, because it offers a high density and good thermal conductivity, compared to those of CVl and PIP process. In general, the fabrication of fiber reinforced SiC matrix composites by reaction sintering involves melt infiltration (Ml) or liquid silicon infiltration (LSI). However, the fabrication of continuous fiber reinforced SiC matrix composites by RS focused in melt infiltration (Ml) such as liquid silicon infiltration (LSl) Vapor silicon infiltration was rarely used for SiC matrix composites. 

The liquid infiltration process (LPI) has to be repeated sometimes due to mass loss and volume shrinkage of the polymer during pyrolysis. Often the liquid polymer is mixed with fillers (passive or active) to inhibit or to compensate shrinkage [252]. 

Historically, the processing routes moved from the isothermal CVI process to more cost-effective techniques such as gradient-CVI and liquid polymer or liquid silicon infiltration. These routes are faster and lead to shorter manufacture cycles than isothermal CVI and, especially the two liquid phase processes LPI and LSI, use technologies already developed for polymer matrix composites (PMC). 

An almost contonporary work to the previous report developed a unique yet simple method of BNNT synthesis with the assistance of nanoporous anodized aluminum oxide (AAO) template though the first template-assisted synthesis of BNNTs was reported earlier. In the former method, thermolysis of a boron precursor was used to generate the nanotubular structure inside the template that had been etched out later using hydrofluoric acid and repeated washing with water, methanol, and acetone. The process of incorporation of the polymer inside the nanochannels of the template is known as liquid-phase infiltration (LPI) technique, which has been successfully improvised to make nanotubes of almost 60 pm length and 200 nm diameter (Figure 20.10). 

Curing primarily refers to the process of solidification of polymer matrix materials. Metal matrix materials are simply heated and cooled around fibers to solidify. Ceramic matrix and carbon matrix materials are either vapor deposited, mixed with fibers in a slurry and hardened, or, in the case of carbon, subjected to repeated liquid infiltration followed by carbonization. Thus, we concentrate here on curing of polymers. 

Model of Dissolution. Based on the results described above, a model for the dissolution of phenolic resins in aqueous alkali solutions 1s proposed. The model 1s adapted from Ueberrelter s description for polymer dissolution 1n organic solvents (.21). In Ueberrelter s model, the dissolution process takes place 1n several steps with the formation of (a) a liquid layer containing the dissolved polymer, (b) a gel layer, (c) a solid swollen layer, (d) an infiltration layer, and (e) the unattacked polymer. The critical step which controls the dissolution process is the gel layer. In adapting h1s model to our case, we need to take into account the dependence of solvation on phenolate ion formation. There 1s a partition of the cation and the hydroxide ion between the aqueous solution and the solid phase. The... 

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

FIG. 18.15 Polymer concentration in the surface layer vs. layer thickness (5 d4 = infiltration layer <53 = solid swollen layer <52 + <5n = gel and liquid layer va c = frequency of the stirrer. After Ueberreiter (1968). 

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