Conductive Paper Containing Nickel Plated Pulp Fiber for Electric Field Shielding

Transcription

1 Transaction Conductive Paper Containing Nickel Plated Pulp Fiber for Electric Field Shielding Takeshi Ueno 1, 2, Shohta Takemura 1, Masahiro Shimada 1, and Takayuki Okayama 2 1 Tokyo Metropolitan Industrial Technology Research Institute, 3-6-1, Azuma-cho, Akishima, Tokyo , Japan 2 Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo , Japan Abstract: A conductive paper for shielding electromagnetic waves was produced by mixing pulp fibers with plated pulp fibers, obtained by subjecting disintegrated pulp fibers to electroless nickel plating to impart conductivity to the surfaces, and subjecting the resultant mixture to papermaking. The plated pulp fibers have surfaces covered with metal, and hence, the hydrogen bonds that pulp fibers generally possess do not contribute to the bonding between plated pulp fibers. For this reason, it has conventionally been difficult to manufacture paper from plated pulp fibers. To overcome this difficulty, a conductive paper was prepared from a mixture of plated and unplated pulp fibers. The conductivity of the formed conductive paper was improved by increasing the plated pulp fiber ratio. As a result, both the surface and volume conductivities increased, thus improving the shielding effectiveness against the electric field components of electromagnetic waves. Furthermore, to improve the electromagnetic shielding performance, conductive layers containing plated pulp fibers and insulating layers containing only pulp fibers were stacked on one another to form a multilayer conductive paper. In conductive paper with stacked conductive layers, increasing the number of conductive layers by two increased the electromagnetic shielding effect from 3 db to 24 db. A conductive paper with three conductive layers achieved a general electromagnetic shielding effect of about 30 db at frequencies of 10 MHz to 1,000 MHz. (Received 2 November, 2011 ; Accepted 27 February, 2012) 1. Introduction Electronic circuits are incorporated in commercially available information devices, medical devices, household appliances, and the like. These electronic circuits must satisfy two requirements with respect to electromagnetic noise. One is electromagnetic interference (EMI) ; the other is electromagnetic susceptibility (EMS). EMI is related to noise emitted from an electronic circuit (such as conducted noise carried through a cable and emitted from an electronic circuit, or radiation noise emitted in the form of an electromagnetic wave and may not adversely affect other electric devices such as television and radio sets. EMS is the ability of an electric product to operate without malfunctioning in an environment with electromagnetic noise (such as conducted noise or radiation noise) from another electric device. The EMI and EMS are collectively called electromagnetic compatibility (EMC). In the production, selling, export, and import of electronic products, the requirements for EMC must be satisfied. Otherwise, active measures must be taken to achieve EMC. For example, noise suppression techniques such as adding electronic parts on the circuit board and covering them with an electromagnetic shielding sheet are used to remove noise [1]. The use of electromagnetic shielding materials has drawn attention as a simple method for shielding electromagnetic waves [2]. There are electromagnetic shielding materials that shield the electric field component, the magnetic field component, or both electric as well as magnetic field components of electromagnetic waves. A highly conductive material is mainly used to shield the electric field component. The principle of shielding the electric field component is that when an electromagnetic wave reaches an electromagnetic shielding material, the surface of the material becomes equipotential to the electric field of the wave. Conductivity is used as an index of the electromagnetic shielding performance. A material with a conductivity of 10 2 S/m or less is used for electrodischarge machining, whereas a material with a conductivity from 10 2 S/m to 10 4 S/m is used for shielding electromagnetic waves [3].

2 A conductive paper is an electromagnetic shielding material that consists of paper as a material, or whose production is based on a papermaking technique. It is a type of functional paper through which an electric current can flow [4]-[7], and is suggested frequently as a shielding material [8]-[12]. It was prepared the conductive paper which performed mixture, paper making of plated pulp fiber and pulp fiber. The paper was evaluated in terms of its electrical conductivity and electrical wave shielding effect. The preparation of conductive paper only from plated pulp fibers has been considered difficult. Therefore, we adopted a method to prepare conductive paper by mixing pulp fiber as a binder with plated pulp fiber, in the papermaking process [13][14]. The objectives of this study are to establish methods for preparing plated pulp fibers and conductive paper, as well as to clarify the relationship between the plated pulp fiber ratio and the electromagnetic shielding effect. 2. Experimental 2.1 Preparation process Preparation process for plated pulp fibers Plated pulp fibers were prepared by disintegrating softwood pulp and then subjecting the resulting pulp to electroless nickel plating. The disintegration process was performed in accordance with the method described in JIS P This pulp was not beaten before electroless nickel plating. Electroless nickel plating was conducted by using the Kanigen method. In this method, a catalytic reaction is used. Hence, after a plating reaction is initiated, it proceeds continuously. Therefore, a metal film with the desired thickness can be formed by controlling the plating time. This plating method is mainly consists of the following three steps. The first step involves a process for sensitizing the surfaces of the fibers. The second step involves an activating process for providing palladium as a catalyst for plating the surfaces of the fibers. The third step involves an electroless plating process to initiate the plating reaction. In this process, palladium first allows the deposition of a nickel film, which then functions as a catalyst. The three steps were conducted by using aqueous solutions exclusive to each step. The conditions for each step are shown in Table 1. Aqueous solutions of PINK, RED, and BLUE SUMER (all solutions manufactured by Japan Kanigen Co., Ltd.) were used in the sensitizing, activating, and Table 1 Process conditions. Process Process condition Disintegration In compliance with JIS P 8220 Drying Drying temperature 105 C, Drying period of 30 min Sensitizing Process Washing Activating Process Plating process Immerse test piece (0.2g) in a five times diluted solution (300mL) of PINK SUMER (Japan Kanigen Co. Composition : HCl, SnCl2 H2Oetc.) and agitate solution for 10 min at room temperature. Immerse test piece (0.2g) in pure water (300mL), agitate water for 10 min at room temperature, and remove test piece from water. Immerse test piece (0.2g) in ten times diluted solution (300mL) of RED SUMER (Japan Kanigen Co. Composition : HCl, PdCl2, etc.), and agitate solution for 10 min at room temperature. Immerse test piece (0.2g) in tentimes diluted solution (300mL) of plating solution, BLUE SUMER (S- 680, Japan Kanigen Co. Compositions : NiSO4 6H2O, NaH2PO2 H2O, etc.), and agitate solutionfor10minat60 C Papermaking mm, Filter paper (No. 1) Hot pressing Temperature : 60 C Pressure : 2 MPa plating processes, respectively. The fabrication process for the conductive paper in this study, including the preparation process for plated pulp fibers, is shown in Fig. 1. As indicated by the figure, a drying step was conducted after each of the sensitizing and activating processes. This is because in a preliminary experiment, the plating reaction did not proceed when the plating step was performed without drying. Before setting the plating conditions for the pulp fibers, the relationship between the plating conditions and the thickness of a plating film was examined [15]. When the films obtained by plating at the respective plating temperatures of 60 C and 75 C were compared in terms of thickness, almost no difference was found. This is presumably because hydrogen was generated in the plating step and affected the plating process. Therefore, in the present experiments, a plating bath temperature of 60 C and a plating time of 10 min were employed as the plating conditions Fabrication process for conductive paper As shown in Fig. 1, the plated and unplated pulp fibers were mixed, after which the resultant mixture was

3 Fig. 1 subjected to papermaking to form a conductive paper. With respect to the plated pulp ratio, the four ratios shown in Table 2 were used. (Plated pulp Volume ratio) = (pulp weight before electroless plating) / (binder weight) Papermaking was performed by a method in which a suspension (31.3 g/l) containing plated and unplated pulp fibers was subjected to filtration (filter paper No. 1) to Sample Conductive paper fabrication process. Table 2 Plated pulp volume ratio [v/v%] 1 Plated pulp ratio. Plated pulp weight ratio [w/w%] A B C D : Calculated as the ratio between the mass of the pulp before plating and the mass of the pulp fibers. =W1/(W1+W2) V = Plated pulp volume ratio W1 = pulp weight before electroless plating W2 = binder weight form a mat. The dimensions of the formed conductive paper were mm. After papermaking, the resultant mat was dehydrated and hot-pressed into the form of a sheet. The conditions for hot pressing were as follows : a heating temperature of 60 C, a pressure of 2 MPa, and a pressing time of 3 min. In addition, a multilayer conductive paper with three conductive layers was also formed by alternately stacking layers of conductive paper and pulp each prepared by papermaking and in the wet state and subjecting the stacked layers to hot pressing. The conditions for hot pressing were identical to those used for the conductive paper with one conductive layer. 2.2 Evaluation methods Elemental analysis Cross sections of the above-prepared conductive paper were observed. The conductive paper was embedded in a resin, after which a cross section of the embedded paper was obtained and polished. The resultant sample was observed under a scanning electron microscope (SEM) (JSM-6610LV, manufactured by JEOL Ltd.) Furthermore, elemental analysis of the cross section was performed by using energy dispersive X-ray spectroscopy ( EDX) ( NORAN System E, manufactured by Thermo Fisher Scientific K. K.) The nickel distribution on the cross section of the sample was also observed with EDX Measurement of conductivity The conductivity of the conductive paper was measured by using a resistivity meter (Loresta, manufactured by Mitsubishi Chemical Corporation). We used the four probe method, which is a general method for measuring the inherent conductivity of a material [16]. Two indices the surface and volume conductivities were calculated from the conductivity and the thickness of the conductive paper. The thickness of the conductive paper was measured with a micrometer (Automatic Micrometer, manufactured by High Bridge & Co., Ltd.) in accordance with the method described in JIS P Measurement of electric shielding effect Various methods have been proposed for measuring the electromagnetic shielding effect [17][18]. In this study, we employed the KEC (Kansai Electronic Industry Development Center) method, which can evaluate electromagnetic shielding effect in a nearby area where the electromagnetic shield is positioned near an electromagnetic noise source. This corresponds to a situation in which an electromagnetic noise is emitted from the wiring of an electronic circuit and an electromagnetic shielding material is attached to the wiring to suppress the noise. In the KEC method, which is diagrammatically shown in Fig. 2, the electric and magnetic field shielding effects are measured using a different antenna jig used for each. The figure shows the KEC method for measuring the electric field shielding effect. For this measurement, a transmitting antenna and a receiving antenna were placed so that they faced each other. An electromagnetic wave was first sent without any conductive paper between the antennas.

4 Fig. 2 Measurement system of the KEC method. The amount of the wave sent in this condition was taken as the incident electric field. Subsequently, an electromagnetic wave was sent with a conductive paper placed between the antennas. The amount of the wave sent in this condition was taken as the transmitted electric field. The ratio between the amount of the electromagnetic wave sent without conductive paper and that sent with conductive paper was defined as the electromagnetic shielding effect. Generally, the electromagnetic shielding effect is expressed in decibels and is calculated from the incident and transmitted electric fields by using the following formula. Electric field shielding effect = -10Log (Transmitted electric power / Incident electric power) An SEM image of the cross section of the conductive paper of sample D is shown in the top panel of Fig. 5. The bottom panel shows the nickel distribution on the cross section. The images confirm that the nickel plating process successfully covered the surfaces of the pulp fibers. 3.2 Conductivity of conductive paper The thickness and conductivity of each of the sheets of samples A to D are shown in Table 3. When the plated pulp fiber ratio was increased, both the surface and volume conductivities increased. The reason for this resides in the fact that the network formed from the plated pulp fibers increases such that the conductivity of the sheet increases. Furthermore, the thickness of the sheet increased as the plated pulp fiber ratio was increased. This is presumably because the plated pulp fibers have low flexibility compared to the pulp fibers, and thus cause the fibers in the sheet to be less likely to collapse even when pressure is applied to the sheet. Fig. 3 Conductive paper (Left : Sheet, Right : SEM image). = -20Log (Transmitted electric field / Incident electric field) 3. Results and discussion 3.1 Structure and analysis of conductive paper An SEM photograph of the surface of the conductive paper of sample C is shown in the right panel of Fig. 3. This confirms that the fibers are deposited on one another. Fig. 4 is an enlarged view of the fibers deposited on one another in a layered manner. Fig. 4 is an enlarged view of the fibers deposited on one another in a layered manner. In this figure, the portion indicated by, in which very small protrusions on the surface were observed, is the plated pulp fiber the portion indicated by is the pulp fiber. An elemental analysis was conducted for each portion revealed that nickel was detected in portion but not in portion. Fig. 4 Elemental analysis and fibers of conductive paper.

5 Fig. 6 Electric field shielding effect of conductive paper. Fig. 5 Cross section of the conductive paper (Top : SEM image, Bottom : Nickel distribution (White)). Table 3 Conductivity of the prepared conductive paper. Fig. 7 Overview of the electric field shielding effect. effect A can be calculated from Schelkunoff s equation as shown below. The multiple reflection loss M is small compared to R and A ; therefore, it is omitted here. (2) 3.3 Electric field shielding effect of conductive paper The shielding effect of the electric field component of the conductive paper is shown in Fig. 6. The electric field shielding effect could be improved by increasing the plated pulp fiber ratio. The reason for this resides in the fact that the conductivity of the sheet affects the shielding effect. The equation used to derive the electric field shielding effect [1] is shown below. (3) Here, f, σ, andt represent the frequency [MHz], volume conductivity [S/cm], and sheet thickness [cm], respectively. By using Eqs. (2) and (3) above, R and A are calculated for samples A to D at a frequency of 100 to 1,000 MHz. (1) Here, SE, A, R, andm represent the effects of electric field shielding, absorption loss, reflection loss, and multiple reflection loss, respectively. The relationship between A, R,andM of Eq. (1) with respect to the electromagnetic shielding material is diagrammatically showninfig.7. The reflection loss effect R and the absorption loss Fig. 8 Electric field shielding effect of conductive paper. (R) : reflection loss effect, (a) : absorption loss effect.

6 The results of the calculations in Fig. 8 show that with respect to samples A to D, R is 15 to 33 db, whereas A is several db. A comparison made between these values confirms that the electric field shielding effect of each of samples A to D is more strongly affected by R than by A. In this experiment, we find that the effect of reflection loss is large. A further increase in the conductivity should increase the absorption loss and improve the shielding effect. 3.4 Shielding effect of multilayer conductive paper To improve the electromagnetic shielding effect with respect to the conductive paper prepared in this study, an attempt was made to increase the number of conductive layers so as to form a multilayer conductive paper. To increase the reflection loss, the number of conductive layer was increased. Photographs of the cross section of a multilayer conductive paper are shown in Fig. 9. The top panel shows a photograph of the cross section of a conductive paper with three conductive layers. Furthermore, the photograph of in the bottom panel shows the same cross section in which the nickel distribution is indicated in white. The above photographs reveal that three conductive layers are formed, and that paper layers are formed between two pairs of conductive layers. This result confirms that a multilayer conductive paper with stacked conductive layers can be formed by individually preparing layers via papermaking, stacking the layers on one another in the wet state, and then hot pressing the stacked layers. Fig. 10 shows the shielding effects for sample D, as well as for conductive papers with sheet thicknesses two and three times the thickness of sample D, respectively, prepared under the same conditions. As can be seen from the figure, when the thickness of the conductive paper was increased, there was no remarkable improvement in the electric field shielding effect. Therefore, an attempt to increase the effect of reflection loss was made as a means to improve the shielding effect of the conductive paper. A method for achieving this is to increase the Fig. 10 Electric field shielding effect as a function of conductive paper thickness. Fig. 11 Electric field shielding effect as a function of the number of conductive layers. Fig. 9 Multi-layer conductive paper (Top : SEM image, Bottom : Nickel distribution (White)). Fig. 12 Propagation of electromagnetic waves in conductive paper.

7 number of conductive layers so as to form a multilayer conductive paper. A multilayer conductive paper with stacked conductive layers was formed by using the conditions for sample D (the smallest sheet thickness 1 in Fig. 10). Fig. 11 shows the electric field shielding effect obtained as a function of the number of conductive layers. When the number of conductive layers was increased by one, the electric field shielding effect increased from 3 to 24 db. The reason for this is described below. The propagation of an electromagnetic wave through a multilayer conductive paper with stacked conductive layers is diagrammatically shown in Fig. 12. The figure shows the case for a conductive paper with two conductive layers. Here, E0 and E2 are the electric fields of the electromagnetic wave entering and exiting, the conductive paper, respectively, and E1 is an intermediate electric field. R is the reflection loss of the conductive layer. The values of E0, E1, and E2 when the electromagnetic wave has passed through the conductive layers are respectively represented as follows [17]. (4) (5) (6) When the reflection loss of the conductive paper with one conductive layer is denoted as R1 and that of the conductive paper with two conductive layers is denoted as R2, the relationship between R1 and R2 is shown below. (7) R2 is 6 db larger than R1. That is, the calculation results show that an increase in the number of conductive layers by one causes the effect of reflection loss to increase the electric field shielding effect by 6 db. This result is consistent with the results of the measurements shown in Fig. 12. From the above, we find that the electric field shielding effect of the conductive paper formed in this study depends on the reflection loss. With respect to the conductive paper with a volume conductivity as small as several S/cm, by increasing the number of conductive layers so as to form a multilayer conductive paper, the effect of reflection loss could be increased, making it possible to improve the electric field shielding effect. Furthermore, with respect to sample D, by increasing the number of the conductive layers to three, a shielding effect of 30 db or more was achieved at 10 MHz to 1,000 MHz. Thus, the resultant sample D could be used as a general electromagnetic shielding material. The above results reveal that for conductive paper formed by nickel plating, a conductive paper with three conductive layers exhibits an improved electric field shielding effect compared to a conductive paper with one conductive layer. That is, the results confirm that when using the same amount of plated pulp, stacked conductive layers in the conductive paper favorably affect the electric field shielding effect. 4. Conclusion A new method for producing conductive paper by using a papermaking technique was proposed. A conductive paper could be produced by mixing pulp fibers with highly conductive plated pulp fibers prepared by subjecting disintegrated pulp fibers to electroless nickel plating and then subjecting the resultant mixture to papermaking. Furthermore, a multilayer conductive paper with stacked conductive layers was produced. By increasing the ratio of the plated pulp fibers to pulp fibers (plated pulp fiber ratio), both the surface and volume conductivities could be increased. The above conductive paper showed improved electric field shielding performance by increasing the plated pulp fiber ratio. A conductive paper with conductive layers and pulp layers stacked on one another showed further improvements in the electric field shielding effect. A conductive paper with three conductive layers achieved an electric field shielding effect of 30 db or more, at a frequency of 10 MHz to 1,000 MHz. This level of performance approximates that of a general electromagnetic shielding material References 1. Clayton R. Paul, Introduction to Electromagnetic Compatibility, Wiley (2006). 2. Y. Seki, Electromagnetic shielding technology and materials, Shi Emu Shi Shuppan (1998). 3. Y. Shimizu, Kougyo Zairyo, 42, 3, 18 (1994). 4. H. Inagaki, Sen i Gakkaishi, 46, 195 (1990). 5. H. Inagaki, Sen i Gakkaishi, 50, 427 (1994). 6. K. Fujiwara, Kami Pa Gikyoushi, 49, 160 (1995). 7. K. Kuboshima, Sen i Gakkaishi, 48, 527 (1992). 8. T. Oya, T. Ogino, Carbon, 46, 169 (2008). 9. T. Nakata, Hoso Gijutsu, 37, 268 (2009). 10. S. Shinagawa, J. Porous Materials, 6, 185(1999).

8 11. T. Kadoya, New functions and engineering of paper, Shokabo, p. 30 (2001). 12. S. Shinagawa, K. Urabe, C. Nagasawa, Japan Tappi Journal, 43, 574 (1989). 13. T. Ueno, S.Takemura, M. Shimada, T. Okayama, Fiber Preprints, Japan,65, 3, 126 (2010). 14. T. Ueno, S.Takemura, M. Shimada, T. Okayama, The 2011 Annual Meeting of the Institute of Electrical Engineers of Japan (2011). 15. T. Ueno, S.Takemura, M. Shimada, T. Okayama, Sen i Gakkaishi, 67, 219 (2011). 16. T. Nakazawa, Electric/electronic materials, Koronasha, p. 145 (2005). 17. Sumitomo 3M anti-noise measures group Anti-noise measures-shielding materials and techniques-measurement/analysis/countermeasures, Sangyo-Joho (1989). 18. E. Hariya, Panasonic Techno Trading, 24, 1 (2003).