Preparation, properties and structure of hard carbon materials for medical applications

Abstract

Glass-like carbons represent a wide family of non-graphitizing carbons, which cannot be converted into graphite, even under a high temperature treatment up to 3000 oC. They are hard carbon materials synthesized by pyrolysis of some polymeric precursors. Due to their relative ease of production and a diverse range of properties, such as high thermal resistance, extreme chemical stability, low density and great hardness compared with other carbons, gases impermeability and high electrical conductivity, these materials have been applied in industry since decades. Moreover, glassy carbons exhibit excellent biological compatibility with blood and living tissues, and therefore they have a high potential for the use in medicine. Nowadays, there is an increasing interest in interfacing glassy carbon microelectrodes with tissues for applications ranging from neural signal sensing and stimulation of brain. Furthermore, recent advances in additive manufacturing have led to the creation of ultrastrong glassy carbon microlattices which can be used as medical implants. Although glassy carbons are highly desirable for many applications and are extensively investigated, their properties such as mechanical or electronic performance as a function of the internal structure and processing are still not fully understood and cannot be predicted. The atomic structure of glass-like carbons is complex and strongly depends on the pyrolysis conditions. The most recent studies have suggested that the structure of glassy carbons consists of fullerene-related building blocks, but up to now there are no commonly adopted model of their nucleation and transformation during the carbonization process. The main aim of this work is to establish preparation-structure-properties correlations of a series of glass-like carbons produced by pyrolysis of polyfurfuryl alcohol at different temperatures and go beyond the previous state of the art. Given the complexity of their structure, that can be regarded as intermediate between crystalline and amorphous, and its sensitivity to the synthesis temperature the detailed characterization of the prepared glass-like carbons requires applications of many experimental techniques and interpretation methods. They are: wide-angle X-ray and neutron scattering, Raman spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, nanoindentaion as well as computer simulations of the atomic structure. The fundamental part of these studies was the analysis of the diffraction results in both, real and reciprocal spaces, in form of the structure factors and the pair distribution functions. Theoretical models of the atomic structure were first described in the frame of the paracrystalline structure, and then classical molecular dynamics simulations were performed for energy optimization of the atomic systems containing topological point defects. The model compatibility with the experimental data was verified by a direct comparison of the model-based calculations and the experimental diffraction data. The use of additional techniques, such as high resolution electron microscopy, Raman spectroscopy, electron energy loss spectroscopy and nanoindentation allowed obtaining detailed information about the local structure, chemical bonding between carbon atoms, and mechanical properties of the investigated materials. It has been demonstrated that the structure of the glass-like carbons at different stages of the carbonization process resembles the curvature observed in fragments of nanotubes, fullerenes or nanoonions. This curvature is responsible for hardness and mechanical strength of the glass-like carbons as well for the formation of porosity. It has been established that the constituent carbon atoms are connected mainly by the sp type bonds.