30 May 2019

CHARACTERIZATION

SEM characterization

Carbon fiber covered with boron doped polycrystalline diamond crystals, is a potential material to be implemented into aerospase industry. Doped diamond coating could avoid electrical issues due to bad weather conditions such as those generated during airplane flights.

To reach a deep understanding of the electrical behaviour, a previous characterization of: i) the carbon fiber surface and ii) the interactions between diamond crystals and one is required. In this sense, scanning  electron microscopy studies (SEM) could  play an important role so the appareance and the morphology of the boron doped polycrystalline diamond coating coverage could be characterized. 

The SEM is a type of technique that employs the electron-beam/atomic interactions, to produce various signals containing information about the surface topography and composition of the analyzed materials. The electron beam is scanned in a raster scan pattern and the position of the beam is combined with the intensity of the detected signal to produce an image. Set of signals produced during the scanning of the electron beam, the secondary electrons are the most common emmited particles from the electron-beam/material interactions (its number depends on specimen topography) which can be detected. 

Moreover and due to the very narrow electron beam, SEM micrographs have a large depth of field yielding depth generating a signature three-dimensional apperance that can be used to understand the surface structure of the sample. Hence, it can be evaluated: i) the morphology ii)  size-shape ratio of boron doped polycrystalline diamond cyrstals, to determine the final appareance of boron-doped CFRP materials.

 

CTEM studies

The quantification of boron atoms from boron-doped diamond CFRP materials is a key aspect in terms of electrical-thermal behaviour ant its becomes meaningful for developing of them, so such materials could open a new research niche into aerospace industry.

Under this framework the electronic microscopy-related techniques have proven their merits in allowing the characterization of boron atoms content inside polycrystalline diamond crystals.

Among  them,  conventional electron transmission microscopy (CTEM) is  particularly  powerful to quantify the boron atoms content. In  this mode, a beam of electrons is transmitted through a specimen to form an image. The image is then magnified and focused onto an fluorescent screen or camera.  The screen or camera shows a constrast originated from differences in thickness or density (“mass-thickness contrast”) or from crystal structure or  crystallographic orientation (“crystallographic contrast” or “diffraction contrast”) so this technique is therefore sensitive to chemical and physical aspects. This opens up a way to develop a methodology to quantify the boron doping content and to establish design rules for improving boron doped CFRP materials either”.

 

HRTEM characterization

The nano-structural characterization of the diamond crystal/carbon fiber interfaces is critical to undertanding the growing process of boron-doped polycrystalline diamond on carbon fiber. With this aim in mind, the electron transmission related techniques are good characterization techniques. In particular, high-resolution transmission electron microscopy (HRTEM) is an imaging mode that shows a direct imaging of the atomic structure of the samples allowing the study materials properties with accurance. The image of an object is obtained by recording the 2D spatial wave amplitude distribution in the image plane. This imaging procedure is also often referred to as phase contrast. Nevertheless the information  provided does not necessarily led us to an intitive interpretation as the images is influenced by aberrations in the set of lenses of the microscope. Despite its limitacion, different aspects as: i) size grains, ii) shape grains, iii) crystallographic phases, iv) distribution grains as well as v)  boron-doped diamond crystals arrengment on carbon fiber surface are revealed at nanometer scale. This information obtained thanks to this technique is relevant to adjust and improve the growing conditions for getting boron-diamond CFRP materials  that could show an exceptional electrical-mechanical and thermal behaviour. 

 

EELS studies

To undestand the perfromance ofthe boron-doped polycrystalline diamond crystals on carbon fiber,  the structural characterization of the boron doped polycrystalline diamond crystals/CFRP interface is crucial to the  comprehend of final properties of such materials.  Other method to be able to examine diamond particles based on spectroscopic techniques is electron energy loss spectroscopy (STEM-EELS). This remarkable spectroscopic technique provides data of both surface plasmons and inner core (C 1s) electrons. Inner core electrons provide information about the K-edge loss spectra resulting from the excitation of C 1s electrons to vacant π*and σ* orbitals; the lower energy state is related to sp2 (reported between 285 and 290 eV), and the higher state is related to sp3 (reported at greater than 289 eV).  Thus, the sp2/sp3 rate can be established, which is important to link  the chemical aspects with the final electrical behaviour.

XPS studies

Boron doped polycrystalline diamond CFRP compounds, are a novel technology to improve the electrical and mechanical behaviour of aerospace materials. Due to these new compound are still a field of research, there are questions to be answered as for example how the relation between diamond crystals/CFRP influences its electrical-mechanical and thermal behaviours. X-ray photoelectron spectroscopy (XPS) could provide the necessary information to optimize the growing conditions.

X-ray Photoelectron Spectroscopy (XPS) is a technique for surface analysis of solid materials. By irradiating a solid surface with an X-ray beam, electrons from different energy levels are excited and some of them escape, being collected by the detection system. The number of electrons and its kinetic energy are recorded, generating a spectrum. The maximum sensitivity depends on factors such as the material under study or the X-ray source. The most used XPS contributions are related to core-level electrons escaping without losing energy what are widely used for chemical qualitative and quantitative characterization which permits identify  the interactions between diamond/CFRP interface and therefore to determine the key parameters. Additional information can be obtained thanks to other contributions than that previously described such as valence band electrons, electrons from core-levels escaping with a certain energy loss (X-ray photoelectron energy loss spectroscopy XPELS) or electrons from auger transitions (X-ray auger electron spectroscopy SAES). It is remarkable that auger transitions phenomenon is higher in low Z number elements as carbon. All of these additional contributions are also crucial and must be taken into account to determine the relation between diamond coating chemical and physical properties for aerospace materials. 

 

RAMAN Spectroscopy

Raman is a spectroscopy technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observedRaman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified. Raman spectroscopy relies upon inelastic scattering of photons, known as or Raman scattering. A source light monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range is typically used, although X-rays can also be used. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. The most used Raman contributions are related to the chemical surface aspects as: i) particle sizes, ii) functional groups and iii) phases of a particular atomic element. In this way, this technique permits to develop a quantification procedure, which allows one to derive the crystalline size information from the Raman diamond patterns, and to analyze how the content of carbon structural fragments  of B-NCD/CF are influenced by growth procedure.