| As a new functional composite, wooden electric heating composites have wide industrialization and market prospect in the area of electric heating products. Also, as a kind of multidisciplinary product, lacking of theoretical research, the composite often has some key problems such as large power deviation in practical production, and uneven heating and unstable electric heating effect in application process. Systematic research on the reveal of the resistance evolving regularity, electric heating effect and its response regularity along the preparation process and under long-term load, would provide direct theoretical reference for improving the power deviation of the composite, stability and uniformity for electric heating.Electric heating membrane and the composite using carbon fiber paper as electric heating material in this paper, was systematically researched on the electric heating response regularity and mechanism about carbon fiber paper of different specifications, glue, agglutination factor and different composite structures under short-term load and long-term overload. And it was innovative that unifilar electric heating element was prepared with the carbon fiber monofilament to further analyze electric heating effect and response mechanism of the composite from a micro scale, and form a more scientific electric heating response mechanism about the composites. The main conclusions were as following.(1)Electric heating effect mechanism of carbon fiber paper: the electric heating effect realizes by the conductive network structures in carbon fiber paper, including carbon fiber monofilament and its lap element. And the contact resistance were around 71.4Ω on average in the monofilaments lapping interface playing a main role in electric heating effect. The middle area of monofilaments had a higher temperature. All the monofilament electric heating element, lap electric heating element and carbon fiber paper had a good linear relationship between temperature rise and loading power. Temperature rise rate of the lap element was the largest.(2)Distribution mechanism of electric heating temperature in carbon fiber paper: uniformity for distribution of electric heating temperature in carbon fiber paper depended on the evenness for distribution of carbon fiber monofilaments, the lap and the monofilaments length. The heating temperature of the monofilaments presented a good unary quadratic equation parabola in the length direction and a good GaussAmp equation curve in vertical direction. The area where the temperature difference was less than 7℃ in the monofilament of 10 mm length was in the range of each around 2.5mm left and right of the middle, and in the range of each around 1.5mm above and under the monofilaments. With increase of the monofilaments length, the uniformity improved significantly in the length direction, but not obviously improved in the vertical direction. In the lapped structure, the temperature gathered to a certain extent around the overlapping point and the area of the 7℃ temperature difference decreased.(3)The electric heating composite(UFP) using melamine modified urea-formaldehyde resin(MUF) had higher drop rate of resistance(DRR) and more excellent anti electrical-thermal coupling effect and response regularity than the one(FRP) using epoxy resin prepreg(PP). UFP showed negative temperature coefficient(NTC) effect of linear correlation and FRP showed segmented NTC and positive temperature coefficient(PTC) effect in the thermal-resistance effect of 20~100℃. In the process of two kinds of electric heating composites overloaded for 13 h with 1000W/m2 power density, both presented the NTC effect and UFP was steadier than FRP in the resistance. But the DRR of UFP which was respectively 4.1% and 1.4% were higher than that of FRP(2.5% and 0.04%) when the power instantaneously cut off and after being cooled. Electric heating layer of the UFP whose interface among wood-glue-carbon fiber paper was stable and its glass transition temperature(Tg) was above 250℃ higher than that of FRP(113℃), presenting better dynamic thermal stability than that of FRP. The low polymer in the electric heating layer after overloaded went crosslinking polymerization in further and the Tg increased. Especially FRP, the effect was remarkable and its Tg increased to 132℃. At the same time, the defects such as the disordered structure and so on in the carbon fiber surface after overloaded could had a further oxidation etching under the electrical-thermal coupling effect, and the phase structures of the carbon fiber tend to be messy, but the thermal radiation performance failed to be weaken.(4)After agglutination of the carbon fiber monofilament in wooden composites, its DRR presented negative growth, the uniformity of surface temperature was dramatically improved. The maximum temperature rise basically appeared after loaded for 5min and kept stable within 40 min. After the unifilar electric heating element was loaded with 20W/m unit power for 13 h, its temperature rise was floating in around 1℃. After being cooled, resistance of electric heating element with MUF was reduced by 2.01%, but that with PP increased by 0.35% on the contrary, which showed that the structure and resistance of the monofilament had a change under current.(5)Both the elelctric heating element with MUF and PP showed the NTC effect in the temperature-resistance effect of 20~100℃ and the DRR was 4.54% and 5.07% respectively in 100℃. Its activation energy for graphitizing could be decreased and the amorphous structures could have a relaxation due to the high current density of 1.30×108~2.34×108A/m2 in the continuous loading process with 10~40W/m unit power. At the same time, the regular and irregular structures in the surface were etched and peeled through oxidation, and the phase structures of the monofilaments electric heating elelment(TCF) tend to be obviously improved, so the resistance changed in the loading process and would also affect the thermal radiation performance.(6)The resistance always presented regular downward trend in the bonding process. In the pressing process, the conductive paths increased between two electrodes and on its bottom in the electric heating layer, and the conductive cross-sectional area magnified, and DRR presented a high linear correlation with unit pressure. In the pressure maintaining process, DRR showed an exponential function relation with time. The influence of unit pressure on final DRR was not significant. Glue spread had a significant influence on DRR showing good linear downward relationship with the glue spread. All the final DRR was in the range of 24~40%. The higher the unit pressure was, the more the temporary and fragile "barriers" formed, and the higher the DRR retention rate was after overload. Similarly, the increase of the glue spread also contributed to the formation of the "barriers", but not conducive to the release of thermal radiation.(7)The surface of wooden electric heating composites showed a heat-transfer law with exponential function between time and temperature, and temperature rise and loading power density presented the linear and power function relationship at the same time. When the electric heating layer was moved forward the core layer, the surface temperature unevenness could reduce 1~2℃, but the temperature rise and surface heating rate were reduced and the bottom temperature increased. The surface heating rate and temperature rise could be improved at the same time by introducing heat-insulatting film and it could be furthest shortened by 3min for that the surface achieved 20℃ temperature rise, and the bottom surface temperature have the highest reduction of 6℃.(8)When wooden electric heating composites were loaded with 500W/m2 power density, normal total emissivity was higher than 0.87, the highest electric-to-radiant power transfer efficiency was 82.27%, and the infrared wavelength of thermal radiation focused on the range of 4~25μm. Along with electric heating layer moving forward the core layer, the electric-to-radiant power transfer efficiency gradually reduced and the largest reduction was 10%, because the electrical-thermal radiation effect had some absorption loss in the "radiation-absorption-reradiation-reabsorption" process. |
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