Preparation Of Mesophase Pitch-Based Carbon Foams And Their Applications

Mesophase pitch-based carbon foams are a novel carbon material with an interconnected microstructure which made carbon foams a promising radar absorber. Meanwhile, activated carbons (ACs) produced from carbon foams had been interested in. In this paper, the main results and significant achievements about the preparation of carbon foams and their applications in microwave absorption and ACs were summarized as follows.1. Preparation of mesophase pitch and its carbon foamsMesophase pitch was produced by two-stage thermal condensation from fined petrol residue. It was found that the temperature in the stage under vacuum had effect on the properties of product and the degree of vaccum in this stage had much effect on the yield. The optimum parameters are:pressure of3MPa, soaking at405℃for3h in the stage under pressure; degree of vacuum of-0.093MPa, soaking at405℃for3h in the stage under vaccum. Mesophase pitch by this process had softening point of287℃, mesophase content100% and the yield of16.1%.Carbon foams with uniform size pore were produced in a high-pressure chamber with the petrol residue-derived mesophase pitch (PRMP) or the synthetic naphthalene-derived mesophase pitch (ARMP) as starting materials. They were carbonized and graphitized by heated to700℃～1000℃to form carbon and graphite foams. The influence of basic properties of the two pitches, foaming and heat treatment parameters on the microstructures of carbon foams were investigated. It was found that PRMP with softening point of287℃had more heavy components, poorer flowability, the higher sensitivity to temperature and higher yield of carbon; ARMP with softening point of256℃had better flowability, wider foaming temperature range and lower yield. An uniform temperature environment is the key to the preparation of carbon foams with the uniform pore size and that the foaming pressure is the main factor for the control of pore size.With ARMP or PRMP as starting material as-grown carbon foams were produced as the following process:mesophase pitch was located in a high-pressure chamber, evacuated and heated at2℃·min-1to330℃～370℃soaking for1h, then the temperature was increased at2℃·min-1to500℃～550℃, soaking for1h at the operating pressures and then cooled to room temperature. With the increase of foam pressure the carbon yield and volume density increased while porosity and pore size decreased and the pore shape was near to circular. At the same foaming pressures, both carbon yield and volume density of as-grown carbon foams from PRMP are higher than those from ARMP.2. Crystal structure and mechanical properties of mesophase pitch-based carbon foamsThe effects of heat treatment temperature (HTT) and foaming pressure on the crystal structure of carbon foams were studied by XRD. It was found that the higher final HTT increased the crystalinity and the (002) peaks of the graphic foams were symmetrical. At the same HTT and foaming pressures, the (002) and (004) peaks of the foams in the xz-plane direction became more intense than those in the xy-plane direction, that is, the orientation along the xz-plane direction was preferred. The d002of graphic foams from APMP was smaller and the crystal size was bigger.The effects of HTT and foaming pressure on the mechanical properties of carbon foams were measured by an Instron1185universal material tester. It was found that the rupture of carbon foams from ARMP or PRMP under press was a layer-to-layer and periodic crack. At the same HTT and foaming pressures, the σ and E in the xz-plane direction were higher than those in the xy-plane and the foams from PRMP exhibited better mechanical properties. With the increase of foaming pressures, the compressive strength (σ) and modulus (E) were increased. On the conditions of the same foaming pressures and starting materials, σ and E increased at the range of500℃～1000℃and decreased at the range of2300℃～2800℃. 3. Microwave absorbing property of MP carbon foams and activated foams sandwich compositesThe slabs of carbon foams treated at600℃～800℃were used as core of sandwich composites, whose ground panels were carbon fiber cloth reinforced composites and face panels were glass fiber cloth reinforced composites. The microwave absorbing properties of sandwich composites were measured at the range of2GHz～18GHz by a HP8510B microwave network analyzer. It was found that MP carbon foams were a lightweight dielectric loss material for microwave absorption. The microwave absorption of sandwich composites composed of xy-plane carbon foams was better than that of xz-plane carbon foams. Sandwich composites composed of10mm or15mm carbon foams treated at700℃and cut in the xy-plane direction showed better microwave absorbing properties. The frequency width of the former below-5dB was17.4GHz and that of the latter was up to15.2GHz; the frequency width of the former below-10dB was3.4GHz and that of the latter was up to10.4GHz.As-grown carbon foams at3MPa were activated by steam at700℃for30m in and60min, respectively. The activated foam slabs with the thickness of10mm cut in the xy-plane direction were used as core of sandwich composites. It was found that microwave absorbing properties of the activated foam sandwich composites became better with the activation time increased from30min to60min. The frequency width of the former below-10dB was10GHz and that of the latter was about11GHz; the maximum microwave reflection loss of the former was-24.79dB at14GHz and that of the latter was-17.76dB at13.4GHz, the peak site of maximum reflection loss moved to the direction of lower frequency.4. Preparation of activated carbons (ACs) from carbon foams and their electrochemical propertiesACs were prepared from small-sized carbon foams (SCF) and powdery carbon foams (PCF) with KOH as activator. Although at the same ratio of KOH/C and activation time ACs from SCF and PCF had similar pore characterization while the former was prepared without the steps such as ball milling, drying and screening. With the SCF as starting materials, ACs with superhigh specific surface area (SSA) were obtained with KOH/C of7:1. SSA of ACs produced at lower temperature (700℃～750℃) was high than that of ACs at higher temperature (800℃～900℃) while the mesopore volume of the former was lower than that of the latter. SSA of ACs at750℃was up to2852m2·g-1, mesopore volume0.87cm3·g-1 and the mesopore content56%; SSA of ACs at900℃is up to2691m2·g-1, the mesopore volume1.48cm3·g-1and the mesopore content66%.ACs were used as the electrode materials for electric double-layer capacitors (EDLC) in an6mol·L-1KOH solution. The electrochemical behavior of EDLC was investigated by the constant current charge-discharge method and cyclic volt-ampere method. It was found that the electrodes prepared from ACs with2637m2·g-1activated at800℃for1h exhibited excellent electrochemical properties. The specific capacitance at0.5mA was322F·g-1, the cycling effieciency98%and the specific capacitance at20mA258F·g-1, which showed good capacity retention at intensive current.