Research On Growth Of Graphene By Bubbling Chemical Vapor Deposition In Molten Metal

Graphene is a two-dimensional crystal of sp~2-hybridized carbon with honey comb structure.Owing to its superior chemical and physical properties,graphene has attracted extensive attention and research interests in both scientific and industrial communities,since it was discovered in 2004.Recently,lots efforts have been made on the industrial applications,which require graphene with both the high quality and large quantity.However,the graphene prepared by conventional methods such as chemical vapor deposition(CVD)and liquid exfoliation cannot meet this requirement.For chemical vapor deposition method,although the graphene produced shows high-quality and large-area(e.g.in hundreds of square meters),the volume of the graphene is still extremely low;While for liquid exfoliation,the advantage of this method is the high volume of the graphene production(e.g.in tons).However,the graphene produced by this approach has high defect density,which is significantly detrimental to its performance in application.In addition,owing to the high volume usage of strong oxidants during oxidation,such as sulfur acid,potassium permanganate,and hydrogen peroxide,this approach has a significant impact on the environment.Hence,the mass production of high-quality graphene represents one of the bottlenecks for the applications of the material.In this dissertation,a bubbling chemical vapor deposition(B-CVD)method is designed for mass production of graphene powder.Using this novel approach,both the high quality and large quantity of graphene are achieved simultaneously.In addition,pure carbon foams are also prepared,and their application in electromagnetic interference shielding is studied.The main contents of this dissertation are listed as follows:1.A bubbling chemical vapor deposition(B-CVD)method is designed for mass production of graphene powder.In this method,hydrocarbon gas is directly leaded into molten copper with an aerator.As a result,bubbles containing precursor gas take form,and the precursor gas dissociates into carbon atoms and hydrogen atoms on the surface of bubbles.Thereafter,graphene takes form on the bubble surface,and then is carried to the top copper surface by bubbles.Finally,graphene on copper surface piles up and then is carried to the collector.With the bubbles containing precursor gas continuously taking form,graphene can be continuously obtained.In an experiment with 1.5 L copper as the catalyst at 1450°C,the production rate of graphene can be as high as 9.4g/h,and the methane conversion efficiency can reach 58%.The produced graphene has much higher quality than that produced through liquid exfoliation method,showing a Raman I_D/I_G<0.05 at the flat areas when the growth temperature is higher than 1250°C.The thickness of graphene ranges from 3 to 40 monolayers,which can be controlled by growth condition.Moreover,the extremely low level of chemicals used makes it an environmental-friendly approach.The natural gas can also be used as the carbon precursor,making it a low-cost production.2.Researches on the growth mechanism of graphene and the evolution of crumpled surface are also conducted.The result shows that most of the graphene is produced on the bubble surface,while there is also a small quantity of graphene produced on molten copper surface.In addition,with the increase of growth temperature and methane concentration,the growth rate of graphene accelerates and the thickness of graphene increases.This result is in accordance with that of conventional chemical vapor deposition.When the temperature increases from 1150°C to 1350°C,the growth rate enhances 5 times and the graphene becomes 4 times thicker;when the methane concentration increases from 0.5%to 2%,the growth rate enhances 3 times,and the graphene becomes 2 times thicker.In addition,with the smaller bubble,the production rate of graphene also enhances.The crumpled surface of graphene is induced by the vibration of copper surface caused by bubbles.With the static copper surface,graphene grown on the copper surface will not be pushed crumpled,and will keep growing thick into graphite.3.The produced graphene is applied in various fields.With the high quality and crumpled surface,graphene shows excellent hydrophobicity and large surface area benefiting its application in water/oil separation.Experiment shows that the graphene has an absorption capacity of 85-165g/g toward different oils and organic solvents.In addition,absorbed oils and organic solvents can be removed by heating absorbed samples above the boiling point.The high quality and large surface area of graphene also make it as a good candidate in nanocomposite.For example,a graphene/polyurethane(PU)composite(Figure 5a-b)with a loading level of 20 wt%shows the volume electrical resistivity as low as 0.05?cm,which is more than one order lower than that from previous research using carbon as a conductive filler and comparable to the commercial conductive polymer using metal particles as fillers.Using this composite(the thickness is 1mm),an EMI shielding effectiveness(EMI SE)higher than80 dB at X band can be achieve.This composite can be also applied as heating coating in wearable devices.A 9-volt battery can drive the coated glove with a temperature of30°C higher than ambient.In addition,porous graphene/WPU composites with density ranging from 50 to 140 mg are also produced.These porous composite are mechanically stable and show excellent EMI shielding effectiveness.4.A light-weight,highly flexible and conductive carbon foam with a 3D interconnected network is prepared by direct carbonization of melamine foam in Argon atmosphere.Conductivity of the foam can be tuned by adjusting the heating treatment procedure and can reach to 190 S/m.The carbon foam has a density of 6.15 mg/cm~3with a porosity of 99.7%.Besides,the foam is highly flexible and it can sustain a repeated deformation and recover.The EMI shielding effectiveness property of the foam is strongly dependent on the electrical conductivity and the thickness of the foam.The EMI shielding effectiveness of the foam with a thickness of 1mm can reach 23 dB at X band,and the specific EMI shielding effectiveness can be as high as~3750dBcm~3/g.Besides,the EMI shielding effectiveness only shows small decrease even after bending,compressing and twisting for 1000 cycles.