Synthesis Of Graphene Oxide And Its Application In Fire Early Warning Sensors

Graphene oxide(GO)has shown great potential for many applications in membrane separation,energy storage and conversion,catalysis,and composite materials with its unique structure and excellent properties,and the large-scale and low-cost synthesis of GO with controlled functional groups is an important prerequisite for its wide application.Among the existing preparation methods,the Hummers method is one of the most promising methods for the large-scale application of GO because of its high yield,relatively low cost,and easy assembly.The preparation of GO using pre-oxidized graphite is a common method to improve the yield.In this thesis,the effect of oxidation temperature on the structure of GO is analyzed using pre-oxidized graphite as a raw material;and based on this,an improved Hummers preparation method for low acid synthesis of GO is proposed;based on these synthesized GO precursors,three different GO-based fire early sensing materials are designed in this thesis,namely graphene oxide/nanographene(GO/G)composite films,fan-folded reduced graphene oxide strips(RGOS)and twist-folded reduced graphene oxide fibers(RGOF),and their structural properties,as well as thermal response mechanisms,were systematically investigated.The main research contents and results are listed as follows.The effects of different oxidation temperatures on the GO structures were investigated.It was shown that when the oxidation temperature was 35 °C + 95 °C,the resulting GO had higher oxygen content,higher carbonyl content,and higher thermal stability;while when the oxidation temperature was 5 °C,the resulting GO had more unoxidized graphene regions,and lower thermal decomposition temperature.On this basis,an improved Hummers method for simple,low-cost synthesis of GO by using much less acid is proposed.By controlling the oxidation temperature to reduce the amount of concentrated sulfuric acid used in the synthesis process,the problem of possible safety hazards due to insufficient acid in the synthesis process was solved.The method can produce GO with the same yield,structure,and properties as conventional methods,and the process can save 40% of sulfuric acid usage and reduce 40% of highly concentrated sulfuric acid waste solution.The GO/G composite films were prepared by compounding nano-graphite(G)with GO obtained from different oxidation temperatures,and the GO/G composite films had a similar thickness and chemical composition.However,the different oxidation degrees of GO in the composite films determined the temperature response performance of the composite films.The response time of GO/G composite film obtained from GO precursors with an oxidation temperature of 5 ℃ is about 148±45 s at 150 ℃,which is due to the lower thermal decomposition temperature of GO produced by low-temperature oxidation in the composite film and the easy thermal decomposition into reduced graphene oxide(RGO)with good conductivity,which forms a connected conductive structure with G.The GO/G composite film produced by oxidation at 5 ℃ has better low-temperature warning sensitivity and is suitable as a fire warning sensor material.GO synthesized by modified Hummers method with less acid was prepared into thin films and reduced by immersion in hydroiodic acid/acetic acid(HI/CH3COOH)system,and RGO strips(RGOS)with a special structure of inner GO wrapped by outer RGO could be obtained by controlling the reduction time.This RGOS has better flame retardancy and hightemperature stability than the GO film.The fire warning sensor is obtained by simply folding the RGOS in a fan shape and a designed circuit.When encountering an abnormally hightemperature environment or flame attack,the fan-folded RGOS can make stable and reliable conductive contact with electrodes through thermal expansion,forming a fast early warning response.The response temperature is 150 ℃ with a response time of ～ 3 s,and the response time to flame is only 0.3 s.This is mainly due to the special structure of RGOS,which combines the good electrical conductivity and flame resistance of RGO with the thermal expansion and reduction of GO,forming a fire warning sensor material with a sensitive temperature response and a very short response time.The RGO fiber(RGOF)is prepared by rolling-folding and twisting the RGO strips with a special structure of outer RGO and inner GO layer,and each RGO strip is stacked in the RGOF and tightly extruded with each other.The RGOF is connected to the designed circuit to produce a fire warning sensor.When encountering an abnormal high-temperature environment or flame attack,the RGOF achieves a fast early warning response due to thermal expansion fracture.The response temperature is 150 ℃ with a response time of ～ 20.2 s,and the response time to flame is about 0.7 s.This is due to the good barrier of the outer RGO layer to gases when the RGOF is attacked by flame or high temperature.The rapid expansion of the RGO layer due to the gas generated by the thermal decomposition of the inner GO layer;and the mutual constraint of the RGOF laminated structure,which results in the fracture of the laminated layer due to the tensile stress of the expansion.RGOF can be easily prepared with longer fibers and can be placed in the area where the warning is needed,thus increasing the monitoring range of RGOF fire warning.RGOF can also respond to abnormally high voltages with a fracturing effect.The starting voltage of RGOF fracture is 4 V and the fracture response time becomes shorter as the voltage increases.This is because the current flowing through the RGOF generates Joule heat when the voltage is applied,and when this heat causes the internal temperature of the RGOF to rise,the internal GO expands by thermal decomposition and causes the RGOF to fracture.This is a new potential application of RGOF in preventing circuit overload and overcurrent.