Introduction
Since the early 18th century, carbon materials have been used in a wide range of industrial, automotive, electronics, and other applications. Carbon materials can be classified into two types depending upon their structure and composition: graphitic carbon and non-graphitic carbon. The former are materials consisting of sp2 bonded carbon atoms forming networks with aromatic units, while the latter include isotropic or anisotropic materials with chemical compositions different from graphite or diamond and various morphologies, such as powder, short fibers, whiskers, foams, and spheres. Non-graphitic carbon materials are known for their excellent mechanical, electrical, thermal and chemical properties, which have enabled them to be used in a range of applications, from microelectronics to aerospace.
During the past decades, the research and development of nano-scale non-graphitic carbon materials have been boosting due to their unique properties and potential use in advanced technologies. At nanoscale, these materials can be divided into two categories: nanostructured materials such as graphene, carbon nano-onions, nanotubes and nanofibers, and non-graphitic nanostructured materials such as carbon nitrides, non-graphitic nanocarbon and PCD. These materials present numerous advantages, including low density, good conductivity, and high specific strength, which can be employed to enhance the performance and efficiency of various devices.
This paper reviews the properties and applications of non-graphitic carbon materials, specifically focusing on clustered or networked materials characterized by micrometer-length dimensions. The properties, structure and morphology of non-graphitic carbon materials such as non-graphitic carbon nanofibers and PCD are analyzed in detail. This paper also describes the development and evolution of carbon materials from their early discovery to application in advanced technologies. Different strategies utilized for surface modification and processing of non-graphitic carbon materials, such as rapid solidification of specialized gaseous precursors, deposition using chemical vapor deposition, arc discharge and laser ablation are discussed. This paper also focuses on the latest trends, challenges, and future perspectives of the non-graphitic carbon materials research.
Classification of Carbon Materials
The design and manipulation of materials with prescribed properties have been explored for centuries. The carbon materials can be classified into two broad families based on their atomic structure; graphitic and non-graphitic. Graphitic carbon materials, also known as amorphous carbon (AC) and diamond-like carbon (DLC), are generally composed of sp2 bonded carbon atoms forming networks with aromatic units, while non-graphitic carbon materials (NGCMs) consist of materials with chemical compositions different from graphite or diamond and various morphologies, such as powders, short fibers, whiskers, foams, and spheres.
Graphitic carbon materials
Graphitic carbon materials comprise a broad category of carbon containing materials which are characterized by their sp2 hybridized carbon structure with a highly directional covalent bonding. Depending on their bulk structure, graphitic materials are categorized as amorphous, microcrystalline and nanocrystalline carbon. These materials possess a high degree of planar, conjugated sp2 hybridization, and therefore possess exceptional electrical, thermal and mechanical properties.
Nanostructured materials, such as graphene and carbon nanotubes, are among the most interesting graphitic materials for the scientific community. Graphene, in particular, has attracted much attention in recent years due to its unique electrical and thermal properties, its abundance of sp2 bonded carbon atoms and its large surface area. Carbon nanotubes are also noteworthy since they are lighter and stronger than steel, yet also highly flexible.
Non-graphitic carbon materials
Non-graphitic carbon materials (NGCMs) consist of materials with chemical compositions different from graphite or diamond and various morphologies, such as powders, short fibers, whiskers, foams, and spheres. The properties of these materials are determined by the structure of their sp3 hybridized carbon atoms. The non-graphitic amorphous carbon (NG-AC) and nanocarbon materials exhibit high hardness, good thermal and electrical conductivity, refractory properties and more importantly, high surface reactivity. The cluster precursors and some higher-order nanostructures of nanocarbon materials are widely considered to invigorate the electro-optic and thermal related performances of NGCMs.
The properties of NGCMs can be further enhanced by surface modification. The presence of various functional groups on the surface of non-graphitic carbon materials plays an important role in controlling their properties. Additionally, the use of non-graphitic carbon nanomaterials can also be beneficial in improving the performance of traditional carbon materials in applications such as energy storage devices and fuel cells.
Properties and Applications of Non-Graphitic Carbon Materials
The unique properties of non-graphitic carbon materials make them attractive for a variety of applications, from microelectronics to aerospace. In the past decades, researchers have developed various approaches for their preparation and surface modification to improve their properties for different applications.
Production
The preparation of non-graphitic carbon materials is often a complex process, which needs to be tailored for a particular application. These materials can be produced by a variety of techniques, such as spark plasma sintering, chemical vapour deposition, arc discharge and laser ablation.
Rapid solidification of specialized gaseous precursors, such as carbon suboxide (C3O2) or hydrocarbons, is an approach that has become established recently in the field of non-graphitic carbon material synthesis. This method can be used to create homogeneous and highly structured material, that can be quickly processed into powders or nanofibers.
Approximately 20 years ago, Uemura et al. prepared the first aligned non-graphitic carbon nanofibers (NG-CNF) with reinforced structures. This method involves the mechanized production of NG-CNFs via a laser-induced pyrolysis foam process. The construction of NG-CNFs entails the rapid progression of a pyrolysis reaction of organic precursor monomers at temperatures close to their decomposition temperature.
Surface Modification Strategies
Non-graphitic carbon materials are not only desirable due to their unique properties, but also due to the fact that they can be subjected to surface modification to suit different applications. Chemical and physical treatments such as coating with silanes, noble metals or polymers are some of the most common and effective surface modification strategies for non-graphitic carbon materials.
Silanes, in particular, are selected in many processes due to their high layer stability and strong electrostatic interactions between silane and the surface of non-graphitic carbon materials. These bondings help to protect the non-graphitic carbon material and prevent the leaching of any species that could cause corrosion, thus increasing its stability.
Noble metals, such as gold or silver nanoparticles, can be used to improve the photocatalytic activity, electrical conductivity and stability of non-graphitic carbon materials. Moreover, the presence of noble metal nanoparticles greatly enhances the optical and electrical properties of non-graphitic carbon materials.
In addition to these physical treatments, chemical agents, including liquid crystals, sol-gel precursors, and surfactants, can be employed in many different processes, such as wet-chemical treatments, composites, and electrolytic treatments. These chemical agents help to modify the surface of non-graphitic carbon materials in order to improve their wettability, electrical conductivity, and mechanical strength.
Applications
Non-graphitic carbon materials have found applications in a wide range of areas due to their unique properties and potential for chemical and physical surface modification. These materials can be used for electrode fabrication for batteries, capacitors and super capacitors, fuel cells, and electrocatalysts. They are also used as support materials for catalysts, leading to improved device performance and stability. Additionally, the use of non-graphitic carbon materials improves the mechanical, electrical, and thermal properties of composites, leading to enhanced wear and corrosion resistance.
Non-graphitic carbon materials also possess a good specific strength and high surface area, which make them ideal for aeronautical and aerospace applications. These materials are often used as reinforcement materials in light-weight aircraft structures and fuel cells. Additionally, they possess high electrical insulation properties, making them essential in the field of microelectronics and optics. These materials can be used as insulating coatings in integrated circuits, lasers, and flat-panel displays.
Conclusion
Due to their unique properties and potential for chemical and physical surface modification, non-graphitic carbon materials have found application in a wide range of industrial and technological fields. NGCMs can be divided into two main categories: nanostructured materials and non-graphitic nanostructured materials. These materials possess high hardness, good thermal and electrical conductivity, and refractory properties. In addition to these, NGCMs possess good wetting behavior, corrosion and mechanical strength, and high surface area which are ideal for different applications.
Different techniques, such as spark plasma sintering, fast pyrolysis and chemical vapor deposition, are used for the production of these materials. In order to tailor them for specific applications, surface modification strategies such as the use of silane, noble metal, polymer or liquid crystal agents, can be employed.
The research and development of non-graphitic carbon materials have increased in past decades due to their unique properties. It is expected that this technology will have a significant impact on advanced materials and applications, from energy storage and fuel cells to microelectronics, optics and aeronautical applications. With an increased use of non
