"Research on carbon" (11): all kinds of "long" carbon and carbon allotropes

In the previous article, we talked about carbon fiber is an organic fiber in the case of keeping its shape unchanged, through solid-phase carbonization, that is, "burning" to remove some of the non-carbon atoms and made. Here, talk about the formation of various carbonaceous materials in another way, through the gas phase carbonization "long".

All kinds of organic compounds and high carbon-containing solids, such as natural graphite and coal, can be used as sources of carbon. After being pyrolyzed into carbon atoms or low molecular compounds by different methods, they can gradually "grow" into various colorful carbonaceous materials. Organic compounds with carbon atoms below 20 have boiling points below 200 ℃. Under normal conditions, most of them are "grown" into carbon by gas phase carbonization. This change is happening around us every day. Look at the bottom of your cooking pot. The black soot (also called carbon black) attached to it is the carbon particles "grown" after incomplete pyrolysis of methane or carbon monoxide in the gas. A 10 nanometer-sized soot is "long" by nearly 50000 carbon atoms. As early as the second century AD, China has known the use of soot ink. The process of making ink by burning loose branches to make soot is recorded in "Tiangong Kaiwo" written by Song Yingxing in the Ming Dynasty. And the emergence of ink has made a great contribution to the preservation of the vast ancient cultural history. Today, carbon black is a large-scale production of industrial materials, it is from natural gas, acetylene and other lower hydrocarbons through pyrolysis or incomplete combustion after gradually "long" into condensed polycyclic aromatic hydrocarbons, and then form carbon. Although there are many processes, they can be completed in an instant, as shown in Figure 1. 90% of the production of carbon black is used in the rubber industry such as tires as reinforcing materials, is also used in plastics, electrical industry, is also used to make ink and paint.

Figure 1. Schematic diagram of carbon black "long".

Carbonaceous materials can also be made of high carbon containing solids, such as natural graphite, anthracite, etc., through arc heating, plasma decomposition, ion sputtering and other physical processes to evaporate carbon into carbon atoms or make organic pyrolysis, dehydrogenation, deposition of the so-called chemical vapor deposition (CVD) method to "grow. In the CVD method, most of the organic matter undergoes a similar path to the generation of carbon black to form graphene fragments; while the carbon atoms directly evaporated into carbon vapor can be polymerized to form a carbon chain, and then the carbon chain will form a carbon ring, and polycyclic polycondensation will also form graphene fragments. These fragments have unsaturated dangling bonds, because of its strong activity to close the formation of carbon 60 such as carbon spheres, carbon spheres can continue to grow into carbon particles, carbon particles will grow up to accumulate chain beads, and eventually form a variety of carbonaceous materials.

Although organic matter will become char during pyrolysis, if the raw materials used, the heating conditions, the deposition location of the carbonaceous matter have no gas-solid interface and whether there are trace metals, especially transition metal particles, during pyrolysis, the carbonaceous material will grow completely different, even kaleidoscope. Now through the "long" method in different temperature ranges and conditions can not only produce carbon black, pyrolytic graphite, diamond-like films and different forms of gas phase growth carbon fibers and other different uses of carbon materials, and even grow a variety of carbon allotropes. Figure 2 shows the different temperature ranges and associated conditions for the "long" formation of various carbonaceous materials.

Figure 2. Different temperature ranges and related conditions for the "long" formation of various carbonaceous materials.

Diamond-like carbon and pyrolytic graphite are used for wear-resistant coatings of hard disks and magnetic heads due to their high hardness, while highly oriented pyrolytic graphite has been used as an essential component in X-ray diffractometers and neutron filters and monochromators in nuclear reactors. Carbon-based composite film materials represented by diamond-like carbon have always been a hot topic in the research of low-friction surface technology. The State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, adopted multifunctional composite vapor deposition technology to develop high-performance carbon-based film technology on the surface of corresponding piston rings, engine tappets, refrigerator pistons, compressor blades or sliding blades, molds and other key components. The carbon/carbon composite material formed by pyrolytic carbon deposition on carbon fiber, carbon cloth, carbon felt and other three-dimensional fabrics has a wide range of applications in aircraft brake pads, aerospace, military and other fields.

In addition, the growth of carbon can be purposefully controlled to produce nano-carbon materials with extremely wide application prospects, such as graphene from sp2 to sp3 hybrid bonding, fullerene such as carbon 60, single-walled and multi-walled carbon nanotubes. figure 3. nanomaterials such as graphene, carbon 60, onion carbon and carbon nanotubes "grown" under various conditions and the proportion of sp2 to sp3 hybrid carbon bonds respectively.

Figure 3. The proportion of sp2 to sp3 hybrid carbon bonds in graphene, carbon nanotubes and other nanomaterials

Figure 4.CVD "growing" graphene (a) carburizing carbon deposition mechanism (B) surface adsorption growth mechanism

The growth conditions are different, even if the same carbon source is "grown" into the same material, the growth mechanism may be completely different. For example, using ordinary methane (12CH4) and carbon 13 isotope labeled methane (13CH4) as raw materials, the growth of graphene on nickel and copper was studied by CVD method at 900~1000 ℃, respectively. The results show that carbon atoms produced by methane cracking can penetrate into nickel matrix with high dissolved carbon content at high temperature. When the temperature is lowered, carbon atoms first come out in single phase dialysis (segregation) on the surface and "grow" into graphene. Then it is precipitated by precipitation (precipitation) to form multi-layer graphene and even superimposed to form graphite, as shown in fig. 4(a) (carbon 12 atoms are black and isotope carbon 13 atoms are red in the figure). On the copper substrate with a low amount of dissolved carbon, the carbon atoms after methane cracking are only adsorbed on the surface, and "grow" into "graphene islands" according to the surface growth mechanism, and then further grow to form a larger graphene film (Figure 4b). The use of CVD to "grow" graphene on a copper substrate is currently considered to be the best method for producing uniform single-layer graphene.

Due to the unique electronic properties of graphene and carbon nanotubes, they are considered to be a new generation of carbonaceous materials that may replace silicon. A series of excellent properties such as electrical conductivity, thermal conductivity, and corrosion resistance are increasingly revealing in the fields of microelectronics and electrochemistry. The new head corner has become the focus of scientific and technological research and development in the world today, and the successful application of various "long" carbonaceous materials has been and is constantly changing our lives.

This article is from Wang Maozhang Science Net blog.
Link address: http://blog.sciencenet.cn/blog-526286-448817.html