Relationship Between Composition,Microstructure And Electrical Resistivity Of SiC Fibers

High-temperature wave absorption depends on making use of the resistive type of wave absorber.It is not the single-layer absorbing structure,but the multi-layer can meet the requirements of advanced absorbing materials.Different layers have different requirements for resistivity.The reflector requires fiber with low resistivity.However,The loss layer and matching layer require fiber with high resistivity.The electrical properties of interlayer fibers need to be reasonably designed to meet the requirements of wave absorbing design.Continuous SiC fiber with adjustable resistivity,high temperature resistance,oxidation resistance and good mechanical properties is an ideal reinforced fiber for wave absorption materlais.This paper analyzes the relationship between fiber resistivity,compostion and structure.Through the regulation of carbon content,achieving the regulation of fiber resistivity with wide range.The stability of high temperature under different atmosphere is tested.KD-I、KD-II、KD-S and KD-SA prepared by National University of Defense Technology is the research object of this paper.At first,mearsure the resistivity of each fiber and then the composition and microstructure of SiC fibers were characterized by elemental analysis,XPS,AES,SEM,XRD,SAXS and so on.KD-I is covered with a50nm carbon rich layer,so the resistivity of KD-I is very low(10^-1～10⁰Ω·cm).KD-II is covered with a 10nm carbon rich layer and the content of free carbon is as high as20.1at%which results to the low resistivity(10^-1～10⁰Ω·cm).KD-S is also covered with a 10nm carbon rich layer,but the content of free carbon is as lower than KD-II,so the resistivity is as high as 10⁰～10¹Ω·cm.KD-SA has the lowest content of carbon on the surface,and the free carbon is comparable to KD-S.However,the porosity of KD-SA is higher than KD-S,so KD-SA has a higher resistivity（10¹～10²Ω·cm）.From what has been discussed above,the carbon-rich layer and the carbon content plays the most important role in fiber resistivity.Meanwhile,the resistivity of these four SiC fibers is low.In this paper,the carbon in fiber is further reduced by decarburization with ammonia and prepare KD-SN1（10³～10⁴Ω·cm）,KD-SN2（10⁵～10⁶Ω·cm）and KD-SN3（10⁸～10⁹Ω·cm）.Finally,achieving the SiC fiber resistivity is adjustable over a wide range(10^-1～10⁹Ω·cm).The effect of high temperature treatment（800-1400℃）in air on the resistivity of SiC fibers is studied.After heat treatment in air,the resistivity change of KD-II and KD-S are small while KD-I and KD-SA have the sudden rise in resistivity.After heat treatment in air for 1h,the resistivity of KD-I goes from 10^-1～10⁰Ω·cm to 10⁵～10⁶Ω·cm.This is mainly due to oxidative damage of the carbon-rich layer on the surface of the fiber and the fiber is mainly composed of a small amount of free carbon,amorphous SiC and SiC_xO_y with high resistivity.Along with the further increase of heat treatment temperature,SiC_xO_y in the fiber start to decompose which causes the resistivity drop down,until heat treatment at 1200℃for an hour,the resistivity drop to 10⁴～10⁵Ω·cm.After heat treatment in air at 800℃for 1h,the carbon content on the surface is also dropped and the interior of fiber is mainly composed of crystalline SiC and porosity which results to the fiber resistivity rises from 10¹～10²Ω·cm to 10³～10⁴Ω·cm.Along with the further increase of heat treatment temperature,the fiber resistivity drop down,until heat treatment at 1200℃for an hour,the resistivity drop to 10²～10³Ω·cm.The effect of high temperature treatment（1000-1800℃）in Ar and vaccum on the resistivity of SiC fibers is studied.In vaccum,the decomposition of SiC_xO_y on the surface results in the increase of the carbon layer’s thickness after heat treatment.After heat treatment in vaccum at 1800℃for 1h,the resistivity of KD-II,KD-S and KD-SA all drop to 10^-2～10^-1Ω·cm.By testing the thickness of carbon-rich layer,the fiber resistivity is similar to the resisitivity calculated by core-shell structure models.The variation of SiC fiber resistivity in Ar is similar to that in vaccum.Along with the increase of heat treatment temperature,the fiber resistivity is dropped down.As the decomposition of SiC_x O_y is more intense in vaccum at high temperatue,the decline range of fiber resistivity is more moderate in Ar.