ELECTRICAL CONDUCTIVITY MEASUREMENT OF FIBERS AND YARNS Veronika ŠAFÁŘOVÁ 1, Jan GRÉGR 2 1 Technical University of Liberec, Faculty of Textile Engineering, Department of Evaluation of Textiles, Liberec, Czech Republic 2 Technical University of Liberec, Faculty of Science, Humanities and Education, Department of Chemistry, Liberec, Czech Republic safarovaveronika@seznam.cz Abstract It is very important to deal with electrical properties of textile products. Textile structures that exhibit conductivity or serve an electronic or computational function are called electrotextiles. They can have a variety of functions, like antistatic applications, electromagnetic shielding (EMI), electronic applications, infrared absorption or protective clothing in explosive areas. In this study, electrical conductivity of different types of carbon fibers was investigated and compared. Further relations between electrical conductivity and other properties of these fibers were found. Key words: carbon fibers, electric resistivity, mechanical properties 1. Introduction In recent years, conductive textile products and textile-based composites have been considered for electromagnetic shielding and anti-electrostatic purposes in various applications for electrical, electronic industries and the defense. This is mainly due to their desirable properties in terms of flexibility, electrostatic discharge, EMI protection, radio frequency interference protection, thermal expansion matching, and weight [1]. Many synthetic fibers that are used in the textile fabrics are insulating materials with resistivity of order of 10 15 Ω/cm 2. This is much higher than the desired resistivity for electromagnetic shielding applications and antielectrostatic purposes. For example, the desired resistivity of an anti-electrostatic material is in the range from 10 9 10 13 Ω/cm 2, for shielding material it is lower than 10 2 Ω/cm 2 [2]. One way of creating interactive electronic textiles can be using minute electrically conductive fibers. They can be produced in filament or staple lengths and can be incorporate with traditional non-conductive fibers to create yarns that possess varying degrees of conductivity. Conductive fibers can be classified into two general categories, those that are naturally conductive and those that are specially treated to create conductivity [3]. Naturally conductive fibers or metallic fibers are development from electrically conductive metals such as ferrous alloys, nickel, stainless steel, titanium, aluminum, copper etc. Other naturally conductive fibers are carbon fibers and fibers made of inherently conducting polymers. Specially treated fibers represent metal coated fibers, fibers filled or loaded with carbon of metallic salts, bicomponent fibers etc. Routes to conductive textile product involve the usage of conductive fibers which was mentioned above. Another route to create conductive textiles is coating of textiles with metals. Silver-, copper-, and nickel-coated textiles using various synthetic fibers are now commercially available. Considering limitations of usage of conductive polymer fibers, textiles of various kinds represent a reasonable choice as a substrate for thin coatings of such polymers. Conductive textile composites based on polypyrrole or polyaniline result in structures showing surface resistances of 10 10 3 ohm/square. The present paper deals with electrical conductivity of different types of carbon fibers. Studied were further relations between electrical conductivity and other properties of these fibers.
2. Carbon fibers Carbon fibers are fibrous carbon materials with carbon content more than 90%. They are transformed from organic matter by 1000 1500 C heat treatment, which are the substance with imperfect graphite crystalline structure arrange along the fiber axis. The existence of carbon fiber came into being in 1879 when Edison took out a patent for the manufacture of carbon filaments suitable for use in electric lamps. The expansion of areas of application for carbon fibers is stimulated by their attractive properties, not found in other materials, such as strength, electrical conductivity, stability on exposure to reactive media, low density, low-tonegative coefficient of thermal expansion, and resistance to shock heating. The most representative applications of carbon fibers are as sorption materials, electrostatic discharge materials, catalysts, and reinforcement materials in composites [4]. 2.1 Classification and types The manufacturing technology for carbon fibers (CNFs) is based on the high temperature pyrolysis of organic compounds, conducted in an inert atmosphere. Some carbon fiber classifications are based on carbon fiber modulus, strength, and final heat treatment temperature. Based on CNFs properties, carbon fibers can be grouped into: Ultra-high-modulus, type UHM (modulus > 450 GPa), High-modulus, type HM (modulus between 350 450 GPa), Intermediate-modulus, type IM (modulus between 200 350 GPa), Low modulus and hightensile, type HT (modulus < 100 GPa, tensile strength > 3 GPa). Based on precursor fiber materials, CNFs are classified into: PAN-based, Pith-based, Mesophase pith-based, Isotropic pitch-based, Rayon-based, Gas-phase-grown carbon fibers. Based on final heat treatment temperature (HTT), carbon fibers are classified into three classes: Partially carbonized fibers (HTT 500 C, carbon content up to 90 wt%), carbonized fibers (HTT 500 1500 C, carbon content 91-99 wt%), and graphitized fibers (HTT 2000 3000 C, carbon content over 99 wt%) [5]. 2.2 Properties of carbon fibers The characteristics of carbon fiber material are influenced by choices of the initial polymer raw material, conditions of carbonization and heat treatment, and also by introduction of certain additives. PAN-based CNFs demonstrate 200 400 GPa Young s modulus upon longitudal stretching of the fiber, mesophase pitch-based CNFs are at interval 150 800 GPa. Carbon fibers have much higher heat resistance than many other known materials. The maximum temperature in air at which the structural and mechanical characteristics still remain unaltered is 300 C. The electrical properties of carbon fibers are explained in terms of π-electron delocalization. It is known that percoalion occurs due to tunneling or hopping of electrons from one fiber to other fiber or by actual formation of a nanofiber network. Variations in composition and structure of CNFs, related to the conditions of their production and to impurities present in their structure, are the cause of changes in electrical resistivity of these materials, resulting in properties ranging from those of conductors to semiconductors. Carbon fibers obtained at lower heat-treatment temperatures pertain to semiconductors, see Fig. 1 [4].
In general, the electric resistivity of CNFs is controlled in three principal ways: - by the time-temperature regime during fiber formation, including the surface impulse heat-treatment; - orientation stretching of the fibers during the formation stage and during carbonization; - the introduction of chemical elements and functional groups into structure of the initial raw materials or the carbon fibers [4]. Figure 1. Comparison of temperature dependence of the electrical resistivity of ex-pan fibers heat treated at various temperatures [4] 3. Electrical characteristics The resistance, R, of a wire of fiber (annular cross-section) with length l can be calculated with the following formula: l ρ 4 l ρ R = =, 2 (1) A π d where R is the resistance of the conductor [Ω], l is the length of the conductor [m], ρ is the electrical resistivity of a conductor [Ω.m], A is the cross-sectional area [m 2 ], d is the nominal diameter of the fiber [mm] [6]. Electric resistivity, ρ, (also known as specific electrical resistance) [Ω.m] is a measure of how strongly a wire opposes electric current. A low resistivity indicates a wire that readily allows the movement of electrical charge. Copper has a resistivity of 0,0171 Ohm mm 2 /m and is, therefore, one of the best conductors for electric current (slightly behind pure silver) [6]. 2 R A π d R ρ = =. (2) l 4 l
A mass specific resistance, R S, is defined as the resistance in ohms between the ends of a specimen 1 m long and of mass 1 kg; the units are Ω.kg/m 2. The two quantities are related as follows: R S = ρ. D, (3) where D is density of material [kg/m 3 ]. In practice, it is more convenient to express R S in [Ω.g/cm 2 ]. With these units, the resistance, R, of an arbitrary specimen is given by the relation: L S 10 N.T 5 R = R., (4) where L is distance between the end of the specimen [cm], N is number of ends of yarn or fiber, and T is linear density of yarn or fiber [tex] [6]. 4. Experimental 4.1 Selected carbon fibers Different carbon fibers continuous tows were chosen for measurement of electric resistivity. Fibers varying in physical, mechanical and electrical properties were chosen from different producers and were based on different fiber precursor materials. The details of investigated carbon fiber continuous tows are listed in Table 1,2. Microscopic images (scanning electron microscope) of rapture of studied fibers are showed at Fig. 2. Table 1. Specification of various carbon fiber tows Code Producer Designation Precursor IMS Toho Tenax IMS60 E13 24K material PAN UTS Toho Tenax UTS50 F13 12K PAN Celion 3000 Celion Carbon Fibers Celion 3000 PAN Celion 6000 Celion Carbon Fibers Celion 6000 PAN K637 Mitsubishi Dialed K63712 Pitch CN80 Nippon Granoc CN-80 Pitch Panex Zoltec Panex 35 50k PAN T700 Toray T700S PAN Code Density [g/cm 3 ] Table 2. Properties of various carbon fiber tows Linear density [tex] Tensile Strength [MPa] Tensile Modulus [GPa] Elongation [%] Electric Resistivity [mω.cm] IMS 1,80 830 5600 290 1,9 1,45 UTS 1,79 800 4800 240 2 2,05 Celion 3000 1,78 200 3400 230 2,2 1,82 Celion 6000 1,78 400 3400 230 2,2 1,82 K637 2,12 2000 2620 630 0,4 0,66 CN80 2,17 890 3430 780 0,5 0,50 Panex 1,81 1500 4137 242 1,5 1,55 T700 1,80 800 2550 135 1,7 1,60
7th International Conference - TEXSCI 2010 September 6-8, Liberec, Czech Republic a) b) c) d) e) f) g) h) Figure 2. Microscopic images of fracture of carbon fibers studied in this paper : a) CN80, b) IMS, c) K 637, d) UTS, e) Celion 3000, f) Celion 6000, g) Panex, h) T700
4.2 Electrical resistance measurement The two probe method was used to test the resistance of carbon fibre continuous tow. Diagram of configuration of the two probe method is shown in Fig. 3. Digital multimeter Agilent 34401A was used. Samples were air-conditioned and measurement was carried out at 40% RH, 22 C. Resistance of samples was measured at different distances between the ends of the specimen (0,05; 0,1; 0,15; 0,2; 0,25; 0,3 m). Linear regression was used for determining the mass specific resistance R S. Electric resistivity of samples was calculated by means of formulas (3). 5. Results and discussion 5.1 Comparison of electrical resistivity Figure 3. Configuration of electrical resistivity measurement of samples In Table 3, there are listed values of a mass specific resistance of samples measured by means of two probe method. Values of electric resistivity of samples are listed too. We can compare these values with values of electric resistivity stated in technical data sheets of producers. Table 3. Measured and declared values of resistivities of samples. MEASURED VALUES VALUES DECLARED BY PRODUCER Code R S ρ stdev (ρ) ρ [mω.g/cm 2 ] [mω.cm] [mω.cm] [mω.cm] IMS 2,17 1,21 0,16 1,45 UTS 3,25 1,82 0,21 2,05 Celion 2,76 1,55 0,04 1,82 Celion 2,71 1,53 0,05 1,82 K637 0,96 0,45 0,05 0,66 CN80 0,75 0,35 0,01 0,50 Panex 1,92 1,06 0,14 1,55 T700 2,55 1,42 0,07 1,60 We can observe that values of electric resistivity of samples declared by producer are higher than values measured by means of two probe method. The difference can be caused by using diverse ways of measurement or using diverse climatic conditions during test. Graphical representation of measured electrical resistivity of samples is on Fig. 4.
Figure 4. Measured values of electrical resistivity 5.2 Resistivity versus tensile modulus and density PAN-based PAN-based pitch-based pitch-based Figure 5. Resistivity versus tensile modulus Figure 6. Resistivity versus density PAN-based pitch-based pitch-based PAN-based Figure 7. Tensile strength versus tensile modulus Figure 8. Density versus tensile modulus
Fig. 5 and Fig. 6 indicate an influence of tensile modulus and density on electric resistivity of samples. We can differentiate two groups of samples - PAN-based samples and pitch-based samples. Dependence of resistivity on tensile modulus is not fully obvious; resistivity is falling down with increasing tensile modulus with samples based on pitch precursor. It is evident, that resistivity of samples is decreasing with increasing density of material. The more and aligned the structural elements (microfibrils) are, the larger is free path for π-electrons movement. As the resistivity decreases, the tensile modulus and the density increases. A graphite monocrystal has tensile modulus of 1050 GPa, density of 2,26 g/cm 3 and reaches the lowest electric resistivity. 5.3 Comparison of other properties Fig. 7 indicates dependence of tensile strength on tensile modulus. We can again differentiate PAN-based samples and pitch-based samples. We can notice that tensile strength is linearly rising with increasing tensile modulus in both groups of samples. The tensile strength naturally depends especially upon fiber surface quality. Tensile strength and modulus can increase together at the same types of material. Diverse types of base materials, producing, and the like there are no relations. Fig. 8 indicates dependence of density on tensile modulus of samples. It is apparent, that density is increasing with growing tensile modulus with pitch-based samples. For reaching higher tensile modulus, micro fibrils, structural elements of fiber have to be more regularly aligned. It means, the higher density, the higher tensile modulus. 6. Conclusion In this research work, method for determination of electric resistivity of carbon fibers was performed. The data got by using two probe method are in good accordance with values of electric resistivity declared by producers. The standard deviation of measurement is comfortable. The influence of different properties of carbon fibers on their electric resistivity was studied in the next step. We can clearly distinguish two groups of samples - PAN-based ones and pitch-based ones. As the tensile modulus of samples increases, the resistivity decreases. Resistivity also decreases with the increase in density of carbon fibers material. The dependences of other physical and mechanical properties were studied lastly. We can again see two groups of carbon fibers with different precursor fiber material. It is obvious, that tensile strength increases with the increase of tensile modulus. As the density increases, the tensile modulus also increases in pitch-based samples. Acknowledgements: This work was supported by the student research grant project 2010 The influence of structure of yarn and content of conductive fibers on electric conductivity of textiles of TUL 7. References 1. PAUL, C.R. Introduction to Elekctromagnetic Compatibility. [s.l.] : John Wiley, 2006. 113 s. ISBN 9780471755005. 2. CHEN, H.C., et al. Comparison of electromagnetic shielding effectiveness properties of diverse conductive textiles via various measurement techniques. Journal of Materials Processing Technology. 2007, 192-193, s. 549-554. 3. LENNOX-KERR, P. Current State of Electrically Conductive Materials. High Performace Textiles. 1990, no. 11, s. 6-7. 4. DONNET, J.B, et al. Carbon fibers. New York : Marcel Dekker, 1998. 567 s. ISBN 0-8247-0172-0. 5. SAVAGE, G. Carbon-Carbon Composites. [s.l.] : Chapman&Hall, 1993. 385 s. ISBN 0412361507. 6. W.E, Morton; J.W.S, Hearle. Physical Properties Of Textile Fibres. London : Woodhead Publishing, 2008. 796 s. ISBN 1845692209.