Freeform fabrication is a type of manufacturing process that allows the fabrication of 3D objects with a large variety of shapes, structure and composition. Laser-assisted chemical vapor deposition (LCVD) is a freeform fabrication technique that allows the deposition of micro-scale structures at a high growth rate with the flexibility of conventional CVD.
The LCVD process
During chemical vapour deposition (CVD), energy is given to a gas (the precursor gas) which contains the atom that we want to deposit. This energy dissociated the gaseous molecules by a series of chemical reactions in order to obtain a solid product (the deposit) on a surface where we want the deposition to occur (the substrate). This can be conceptualized by the following equation which describes the deposition of the gaseous species ABu :
ABu → A(↓) + u B(↑)
The energy can be given to the gas by many different means: heating of the substrate, plasma, lamp, laser, etc. Chemical reactions can occur in the gas itself (homogeneous reactions) and at the surface of the substrate (heterogeneous reactions). The two most important parameters in a CVD process are temperature and pressure since they strongly influence the growth rate. Two possible cases can occur: the growth can be controlled by surface reactions (kinetically controlled regime, the growth rate increases with temperature) or by diffusion (mass-transport controlled regime, the growth rate increases with pressure).
LCVD has been used to produce a large variety of products (fibers, thin films, microcoils, band-gap structures, etc.) made also from a large variety of materials (C, B, BN, W, WC, Si, SiO2, Ti, TiN, Mo, etc.).[1,2,3,4] As for conventional CVD, the advantage of this method over the other microfabrication processes is its ability to use many precursor gases without the need to modify heavily the deposition system. It is also possible to obtain very high growth rates compared to other processes. Indeed, while the deposition surface is small, linear growth rates up to 0.3 mm/s have been reported for the growth of carbon fibers by LCVD. LCVD is also a freeform fabrication process which allows the direct generation of 2D or 3D structures without any additional patterning steps. The two most used LCVD techniques are direct writing, where the laser beam is scanned in 2D on the surface of the substrate, and 3D-LCVD, where the laser focus is moved in 3D above the substrate after the start of growth on its surface. It is also possible to focus the laser slightly above the surface of the substrate in order to induce homogeneous deposition in the gas.
The use of LCVD to deposit cylindrical carbon rods from methane and ethane was reported in 1972 by Nelson and Richardson. Leyendecker and Bäuerle then improved the process by using an Ar+ laser and additional gases, such as ethylene and acetylene. It became clear that the strong temperature gradient induced at the tip of the rod influenced the shape and structure of the deposits. Indeed, the normal growth rate is maximized at the center of the tip and decreases as we go further from this point. This creates radially different growth conditions. On the basis of observations of the surface of the rods and of polished longitudinal sections with an electronic microscope, it was then suggested that the rods were made of pyrolitic graphite with a columnar layered structure. Investigation of LCVD-drawn carbon lines by Raman spectroscopy showed that the surface of the carbon lines (covered by nodules from homogeneous nucleation) consisted of glassy carbon and the interior of ordered pyrolitic graphite.
In the case of LCVD, the laser beam is focused (beam diameter of w) on the substrate and oriented orthogonally to the surface of the substrate. It induces a temperature rise ΔT(r):
ΔT(r) = ΔT0 exp(r² / w²)
where ΔT0 is a constant function of the parameters of the laser beam. Since the pressures used for this project where atmospheric or sub-atmospheric, we were theoretically in the kinetic regime and the linear growth rate R can be written like this:
R(r) ~ exp(-Ea / k T(r))
where Ea is the global activation energy of the deposition reaction and k the Boltzmann constant. When the pressure or temperature increases, homogeneous reactions are starting to happen in the gas and this approximation is no longer valid.
According to previously published experimental and theoretical results on LCVD grown carbon fibers,[2,7,8] the growth rate increases with laser power and pressure while the diameter of the fibers increases with laser power but is independent of pressure. These relations are valid for atmospheric and sub-atmospheric pressures. When the pressure is higher (HP-LCVD), it was reported that the diameter of the fibers decreases with increasing pressure. This is happening because of the cooling effect of the fast movement of the gas near the fiber. Indeed, since the growth rate is very high at high pressure, a convection phenomenon is going on near the deposition area: the gas must quickly fill the space that has been emptied by the deposition. A good part of the heat produced by the laser is then evacuated in the gas before the completion of the deposition, which reduces the size of the reaction zone.
Few theoretical models for the growth of LCVD fibers are available. Most models are very complex, are limited to specific deposition conditions and don’t allow clear results that resemble the experimental observations.[2,10] However, from Arnold’s model, we can gather that the deposition of the fibers is occurring by the accumulation of layers at the tip of the fiber and that once the reaction is started, it no longer depends on the substrate’s nature. Figure 1 illustrates a deposition model for LCVD fibers. The layers have a elliptic or parabolic shape that can be observed with an electronic microscope (SEM) when a fiber is fractured (see Figure 2 and Figure 3).
Characterization of the microstructure of LCVD deposits
Research on LCVD and its possibilities has been pursued for more than 20 years and it had become important to characterize the internal structure of the deposits in order to achieve a better understanding of the process. Investigation of the cross-sectional microstructure of carbon fibers grown by LCVD using Raman spectroscopy has been done. During the first part of the project, a radial Raman analysis was done on a single fiber. Spectra were taken at regular intervals to cover the whole diameter of a fiber. Key parameters of the Raman spectra were studied (G, D, D’ peaks and D/G intensity ratios) and provided insight on the microstructure of the fibers. Indeed, it was discovered that the microstructure changed radially with a well-graphitized center, a nanocrystalline graphite edge and an amorphous carbon surface.
The second part of the project, involved the same kind a radial Raman analysis but several fibers, grown at different laser powers and precursor gas pressures, were studied. This deepened our understanding of the fiber’s microstructure and allowed the discovery of experimental parameters dependencies:
- An increase in precursor gas pressure, increasing the linear growth rate, was accompanied by a decrease in graphitization of the fiber;
- An increase in laser power, increasing the growth temperature and linear growth rate, first induced an increase in graphitization followed by a decrease in graphitization as the growth rate becomes too high.
A turning point in the graphitization level of the carbon fibers was thus observed when the laser power is increased. The knowledge gathered during this research project allows the growth of carbon fibers with desired microstructural properties and further comprehension of the growth process.
Mechanical properties of LCVD deposits
A second project was begun to investigate the local and bulk mechanical properties of LCVD fibers. The goal was to directly relate the mechanical properties to the fibre microstructure.
The investigation of the local mechanical properties was achieved by radial nanoindentation of the fiber cross-sections. The bulk mechanical properties were determined through three-point microbending experiments carried out with the use of a micromanipulator. A clear mechanical properties dependence on experimental parameters was found. The mechanical properties investigated (Young’s modulus, the hardness, the tensile and fracture strengths as well as the bending resistance) will help us determine the type and the extent of applications.
Nanoindentation is a near surface characterisation technique used mainly to determine the mechanical properties of thin films. A sharp indenting tip moves deeper in a material as the applied force is increases and the apparatus measures the degree of response of the material. Load-displacement curves are used to determine the materials stiffness and hardness.
In the case of LCVD produced fibers, a radial change in mechanical properties was observed. The fibers produced at atmospheric and sub-atmospheric pressures have low hardness and elastic modulus. The elastic modulus decreases, from edge to center, as the microstructure changes radially from nanocrystalline to more graphitic structure. The local mechanical properties are directly related to the microstructure induced by the Gaussian laser intensity distribution. Varying the deposition parameters showed that the local elastic modulus and hardness increases with amorphitization. This is achieved by increasing the precursor pressure and by increasing the laser power above a specific value.
Bulk mechanical properties investigation was done by micromechanical testing using a high-precision micromanipulator. During three-point bend testing (Figure 4), the fibers showed an elastic response with no residual strain upon unloading. The low pressure LCVD fibers have poor mechanical properties when compared to other forms of carbon and when compared to HP-LCVD or other commercially produced carbon fibers. It is believed that the fiber’s structural weakness is due to their layered graphitic nature. Again, the mechanical properties improve with amorphization. Internal structural defects due to difficult process control occur at higher pressures and laser powers. Furthermore, a rough surface morphology, due to the presence of surface nodules could lead stress concentrations. This in turn would reduce the overall strength of the fibers.
Laser assisted surface-bound growth of carbon nanofibers
This poster reviews the recent advances in laser assisted surface-bound growth of carbon nanofibers:
Recent advances in laser assisted surface-bound growth of carbon nanofibers: from vertical nanofibers arrays to thin films and nanocomposite coatings (pdf)
Carbon nanofibers and nanotubes are grown by selectively heating catalytic metal nanoparticles in the presence of a gaseous hydrocarbon precursor with a low power (1-5 W) argon ion laser beam (488 nm). The nanofibers are the result of atomic carbon self-assembly initially into graphene sheets, which are essentially large planar 2D molecules, and subsequently into various arrangements of graphene sheets.
The prime innovation reported herein deals with the improved control over nanofiber characteristics during their synthesis and their assembly into various surface coatings. Assemblies in the form of thin nanofiber films, vertically aligned nanofiber arrays, horizontally aligned nanofiber mats and carbon-carbon nanocomposite coatings were synthesized locally on the surface of a porous alumina substrate using a nickel catalyst. Scanning electron microscopy and Raman spectroscopy were used to characterize the different assemblies whereas individual nanofiber texture and nanotexture were assessed by transmission electron microscopy.
Irradiation time ranges leading to the formation of specific assemblies have been identified and are presented. The different pre- or post-synthesis strategies used to control diameter, location and alignment are described. Also, the growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure by choosing the appropriate process parameters are provided.
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