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Generation of hard X-ray radiation using the triboelectric effect by peeling adhesive tape
Journal of Electrostatics 71 (2013) 905e909
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Generation of hard X-ray radiation using the triboelectric effect by peeling adhesive tape Hartmut Stöcker a, *, Maximilian Rühl b, Anett Heinrich a, Erik Mehner a, Dirk C. Meyer a a b
TU Bergakademie Freiberg, Institut für Experimentelle Physik, Leipziger Str. 23, 09596 Freiberg, Germany TU Dresden, Institut für Fertigungstechnik, George-Bähr-Str. 3c, 01069 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2013 Received in revised form 3 July 2013 Accepted 12 July 2013 Available online 24 July 2013
The triboelectric effect describes electrical charging when bringing different materials into contact. We report on the generation of hard X-ray radiation by peeling various adhesive tapes under medium vacuum conditions. Beside vacuum housing and pumps as instrumentation only an electric motor, two rolls and a metal foil as target material are necessary. The spectral distribution of generated X-rays was analyzed using an energy-dispersive detector. Depending on peeling speed, pressure and choice of material combination, electrons with energies sufficient to excite emission in the hard X-ray region are produced. The results are discussed in terms of triboelectric separation of charge carriers. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: X-ray generation Triboelectric effect Adhesive tape
1. Introduction To meet the demands of medical imaging as well as scientific methods for composition and structure investigations, generation of X-rays using miniaturized radiation sources is an interesting field of research. Beside approaches based on the ionizing and electron accelerating properties of high electric fields around pyroelectric crystals [1] also the use of a tribomicroplasma generated in the vicinity of the peeling point of two different polymers is promising [2e5]. The miniaturization of an appropriate setup could be used to engineer portable X-ray sources for the above applications. The triboelectric effect describes an electrical charging of different materials brought into contact [6]. The polarity and strength of the charges depend on the materials chosen and their surface roughnesses. Polymers are typical examples of materials that can acquire such charging [7,8]. If a high density of surface states is available and the work functions FA and FB of the two materials differ, contacting them leads to a band bending (see Fig. 1). Thus, electrons and possibly also ions [9] are transferred across the interface and charges build up on the respective polymers. Sign and amount of charging are described empirically by the so-called triboelectric series [10]. Thanks to this effect, peeling of the two polymers leads to an electric voltage and to the generation
* Corresponding author. Tel.: þ49 3731392773; fax: þ49 3731394314. E-mail address: [email protected] (H. Stöcker). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.07.006
of a plasma at the peeling point [11,12]. Hence, charged carriers are accelerated in the electric field within the gap. If these electrons or ions hit a target, X-rays and other forms of electromagnetic radiation are generated [13e17]. The present study aims to find an optimum set of parameters for X-ray production by peeling adhesive tape. Therefore, different materials and types of contacts will be tested and peeling speed as well as vacuum pressure varied. 2. Materials and methods The setup consists of a vacuum chamber made of glass and stainless steel that houses two spools, with one driven by an external motor with variable speed and the other one rotating freely. A turbo molecular pump with membrane pre-pump evacuates the vacuum chamber to pressures between 102 and 104 mbar. A titanium foil acting as electron target to produce characteristic X-rays is fixed underneath the rolls holding the adhesive tape. An energy-dispersive silicon drift detector (Röntec, Berlin) is connected to the chamber through a thin Kapton window for registering the produced X-rays with minimal absorption losses. Within the chamber, the adhesive tape can be fixed in two ways: Firstly, a full roll of tape is mounted on the freely rotating spool and peeled off by reeling it on the other spool using the external motor. This has the disadvantage of measurement time being limited by the length of the tape. Therefore, secondly, a closed piece of tape is connected around both spools in such a way that the adhesive tape
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H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 1. Electronic band scheme at the interface of two polymers A and B before (a) and after contact (b). Work functions F and Fermi energies EF as well as occupied surface states are indicated. When a contact is established, the high density of surface states leads to band bending and a potential difference.
is running infinitely around the spools when starting the motor (see Fig. 2). This latter setup was used for the present investigations. Several types of commercially available adhesive tape have been used. Details and sample labels are given in Table 1. Apart from X-ray generation tests, surface energy and adhesion force measurements were performed. Surface energies were evaluated from contact angles measured by a Dataphysics OCA 15 EC. Adhesion forces were determined by tangential pulling using a spring scale. 3. Results 3.1. Surface energies and adhesion forces Surface energies were calculated from measured contact angles so that separation of disperse and polar contributions was possible. The results for the samples are given in Fig. 3. The disperse contributions are relatively small, especially for the adhesive sides of the tape. The latter point is apparently desirable for good adhesion of the tape. Comparing the polar contributions of carrier and adhesive side, sample Te1 shows only a small difference whereas the other three samples exhibit bigger and very similar differences. The adhesion forces between carrier and adhesive material measured by a spring scale are given in Table 2 for the four samples investigated before. Sample Te1 exhibits the highest adhesion and Sco1 the lowest. A direct relation with the surface energies is not obvious; however, the electrostatic part of the adhesion force is correlated with the polar part of the surface energy (see Discussion). 3.2. Triboelectric generation of X-rays The X-ray spectra obtained by peeling adhesive tape in vacuum feature a broad background of Bremsstrahlung and several characteristic X-ray emission lines (see Fig. 4). The most intense line is of course related to the titanium foil that was fixed as an electron target underneath the adhesive tape (see Fig. 2). Further lines are related to the components of the vacuum chamber (stainless steel, O-rings etc.) that are as well excited to produce X-ray fluorescence. This was verified by measurements without the Ti foil.
Comparing the spectra obtained for different peeling velocities (see Fig. 5), sample Sco2 exhibits a clear intensity maximum for an intermediate velocity of 10 cm/s. The maximum obtainable X-ray energy was found to be 79 keV at a pressure of 103 mbar and a velocity of 10 cm/s. The maximum X-ray intensity is found in the energy range between 2 and 10 keV. Plotting the X-ray count rate over time reveals quite different curves depending on peeling velocity (see Fig. 6). Nevertheless, all curves show a negative slope, i.e. a steady decrease of X-ray intensity over time is observed. This is mainly related to the degradation and detaching of the adhesive polymer. The longest time of continuous X-ray generation for sample Sco2 was measured as 55,710 s (approx. 15.5 h) at a pressure of 103 mbar and a velocity of 8 cm/s. For all of the samples listed in Table 1, X-ray generation was tested for different peeling velocities and vacuum pressures. The three samples with the strongest X-ray output are compared in Fig. 7. The two Scotch samples are delivering the highest X-ray intensity. A maximum integral intensity of 2.8 million counts was obtained with material Sco2 at a pressure of 103 mbar and a velocity of 10 cm/s, which relates to a count rate of 519 cps. Decreasing the pressure to 104 mbar provides slightly lower optimum intensities but increases the X-ray output at lower velocities. 3.3. Dependence on the type of contact In addition to studying adhesive-carrier contacts (see previous section), also contacts with metals were investigated. The contact between adhesive side of the tape and the different metals was established by using metal foils on the spools or the spool itself that is made of stainless steel. For contacting the carrier side with a metal, an additional free spool was installed within the vacuum chamber and pressed against a roll of tape on the existing driven spool. This setup, employing an additional spool pressing against the driven spool allowed to study also contacts between two adhesive sides as well as between two carrier sides of adhesive tape. Nevertheless, no significant X-ray output was found for these two combinations. For the polymeremetal contacts, it is found that for all three metals, the contact with the carrier side of the tape provides higher X-ray intensities than with the adhesive side (see Fig. 8). Comparing the metals, aluminium clearly provides the highest output showing additionally a slight intensity increase with rising peeling velocity. Finally, copper provides slightly higher X-ray intensity than steel, depending on peeling speed. Table 1 Sample list.
Fig. 2. Schematic setup for triboelectric X-ray generation by peeling of adhesive tape.
Sample
Manufacturer
Colour
Material
Width
Pri1 Sco1 Sco2 Te1
Printus Scotch Scotch Tesa
Transparent Transparent Milky Transparent
Polypropylene Polypropylene Cellulose acetate Polypropylene
19 19 19 15
mm mm mm mm
H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 3. Measured surface energies for the four samples divided in carrier side (C) and adhesive side (A) of the tape.
4. Discussion Using DLVO theory [18] and regarding the peeling point as plate capacitor with dielectric leads to a description for the adhesion force FA as sum of van der Waals force FW and electrostatic force FE: FA ¼ FW þ FE. The electrostatic force can be evaluated from the measured quantities by FE ¼ gp/gtot$FA where gp is the polar part of the surface energy and gtot the total surface energy. Additionally, for a plate capacitor FE ¼ Q$E ¼ Q$U/d is applicable using charge Q, electric field E, voltage U and plate distance d. Using the equation E ¼ Q/(ε0εrA) for the electric field with the dielectric constant ε0 and the sample permittivity εr, finally leads to an expression for the expected voltage:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gp F A d 2 U ¼ gtot ε0 εr A
(1)
Obviously, the assumption of the peeling point being a parallel plate capacitor is a drastic simplification and, additionally, the determination of A and d for any given setup is not straightforward. Nevertheless, this approximation gives a good idea of the relation between adhesion and triboelectricity and how to optimize the material properties for maximum electron acceleration voltage. Namely, a high ratio gp/gtot and a high adhesion force FA are advantageous. Unfortunately, this derivation is only valid under certain circumstances and not in accordance with experimental observations. As shown in the Results section, adhesion forces above 3 N (samples Pri1 and Te1) lead to a reduced X-ray production. This is explainable because a high adhesion force leads to detaching of the adhesive from the carrier polymer so that the X-ray generation
907
Fig. 4. Assignment of characteristic X-ray emission lines in the generated spectrum for sample Sco2 at a velocity of 12 cm/s and a pressure of 103 mbar accumulated for 90 min.
decreases quickly after starting the experiment. The model used for Eq. (1) neglects this unwanted delamination mechanism so that simply maximizing the adhesion force is finally not the ideal choice for triboelectric X-ray generation. Regarding the surface energies, the experiments suggest that a high difference between the surface energies of carrier and adhesive side of the tape leads to the highest X-ray output. Since a high polar contribution of surface energy is favourable for generation of electric fields, the comparison can also be carried out using gp alone. Then, it is found that a difference of approx. 9 mN/m between carrier and adhesive polymer is optimal for our specific setup. Discussing the different types of contacts, it is helpful to rely on the simple model of Fig. 1. It is obvious that no potential is formed between two equal polymers. In accordance with that, no X-ray intensity was observed when contacting equal polymers (adhesiveeadhesive and carrierecarrier). Nevertheless, only small deviations of the electronic surface state occupation are necessary to produce charging between polymers [19]. Replacing one polymer by a metal can result in remarkable Xray output. The difference between the emissions of the metal contacts is explainable simply by their work functions. Contacting a polymer with a metal, a situation similar to the scheme given in Fig. 1 results with the metal being material A (left side). Since the surface states of the polymer adjust to the Fermi level of the metal,
Table 2 Adhesion forces measured between carrier and adhesive side of the tape. Sample
Adhesion force (N)
Pri1 Sco1 Sco2 Te1
3.1 0.9 1.8 3.8
0.2 0.2 0.2 0.2
Fig. 5. Dependence of X-ray emission spectrum on the peeling velocity for sample Sco2 at a pressure of 103 mbar accumulated for 90 min.
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H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 6. Dependence of total X-ray count rate on time and peeling velocity for sample Sco2 at a pressure of 103 mbar.
intensity than copper or steel with work functions of 4.6 eV and 4.7 eV, respectively [20]. It has to be noted that the highest occupied surface state of the polymer may change between subsequent contacts [21], so that an uncertainty range for the work function has to be taken into account. Also, the generated X-ray intensity is modified by the different X-ray yields of the elements acting as target material. Nevertheless, for our setup with additional Ti target foil producing most of the X-ray output, this effect is greatly suppressed. Since the X-ray output depends on the difference between work function of the metal and highest occupied surface state in the polymer, a method for determining this state can be proposed: A metalepolymer contact that does not produce X-rays during peeling relates to a flat band situation at the interface and, hence, the highest occupied surface state of the polymer is equal to the (known) work function of the metal. From our experiments, we can therefore conclude a highest occupied surface state in the polymer at approx. 5 eV below the vacuum level. Since the X-ray output of polymeremetal contacts is always higher for the carrier side, we find that the level of the highest occupied surface state for the adhesive side is above this level at the carrier side. 5. Conclusions
Fig. 7. Comparison of total X-ray counts for different materials, vacuum pressures and peeling velocities during an accumulation period of 90 min. The intensity errors were calculated from 3 consecutive runs.
the work function of the metal defines the potential difference at the interface. For low work function FA, a high potential results and vice versa. Therefore, aluminium with a work function of approx. 4.1 eV produces a higher potential and a higher X-ray
We have shown that the generation of X-rays using adhesive tape works best if the difference between the polar part of the surface energies of the two polymers is in the range of 9 mN/m for the described setup. Additionally, if the adhesion force crosses a maximum of 3 N, the X-ray generation is suppressed by detaching of the adhesive from the carrier polymer. For sample Sco2 at a pressure of 103 mbar and a velocity of 10 cm/s the highest photon count rate of approx. 500 counts per second was observed. Also metalepolymer contacts were found to deliver X-ray output depending on the work function of the metal. This observation could possibly be used to determine the highest occupied surface state of the polymer. Appendix. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.elstat.2013.07.006. References
Fig. 8. Comparison of total X-ray counts for three different metals in contact with carrier or adhesive side of sample Sco1 measured in dependence on peeling velocity at a vacuum pressure of 102 mbar during an accumulation period of 40 s.
[1] J.D. Brownridge, Pyroelectric X-ray generator, Nature 358 (1992) 287e288. [2] C.G. Camara, J.V. Escobar, J.R. Hird, S.J. Putterman, Correlation between nanosecond X-ray flashes and stick-slip friction in peeling tape, Nature 455 (2008) 1089e1092. [3] C.G. Camara, J.V. Escobar, J.R. Hird, S.J. Putterman, Mechanically driven millimeter source of nanosecond X-ray pulses, Applied Physics B 99 (2010) 613e617. [4] S. Kneip, Applied physics: a stroke of X-ray, Nature 473 (2011) 455e456. [5] S.J. Putterman, C. Camara, J.V. Escobar, J. Hird, Mechanoluminescent X-ray Generator, US patent 2011/0130613, 2009. [6] P.A. Thießen, K. Meyer, G. Heinicke, Grundlagen der Tribochemie, AkademieVerlag, 1967. [7] E. Németh, V. Albrecht, G. Schubert, F. Simon, Polymer tribo-electric charging: dependence on thermodynamic surface properties and relative humidity, Journal of Electrostatics 58 (2003) 3e16. [8] V. Albrecht, A. Janke, E. Németh, S. Spange, G. Schubert, F. Simon, Some aspects of the polymers’ electrostatic charging effects, Journal of Electrostatics 67 (2009) 7e11. [9] L.S. McCarty, G.M. Whitesides, Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets, Angewandte Chemie International Edition 47 (2008) 2188e2207. [10] J. Henniker, Triboelectricity in polymers, Nature 196 (1962) 474. [11] K. Nakayama, Microplasma generated in a gap of sliding contact, Journal of the Vacuum Society of Japan 49 (2007) 618e623.
H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909 [12] K. Nakayama, Triboplasma generation and triboluminescence: influence of stationary sliding partner, Tribology Letters 37 (2010) 215e228. [13] E.N. Harvey, The luminescence of adhesive tape, Science 89 (1939) 460e461. [14] V.V. Karasev, N.A. Krotova, B.W. Deryagin, Study of electronic emission during the stripping of a layer of high polymer from glass in a vacuum, Doklady Akademii Nauk SSSR 88 (1953) 777e780 (in Russian). [15] A.J. Walton, Triboluminescence, Advances in Physics 26 (1977) 887e948. [16] M. Zhenyi, F. Jiawen, J.T. Dickinson, Properties of the photon emission accompanying the peeling of a pressure-sensitive adhesive, Journal of Adhesion 25 (1988) 63e77.
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[17] J. Horvat, R.A. Lewis, Peeling adhesive tape emits electromagnetic radiation at terahertz frequencies, Optics Letters 34 (2009) 2195e2197. [18] W.B. Russel, D.A. Saville, W.R. Schowalter, Colloidal Dispersions, Cambridge University Press, 1989. [19] J. Lowell, W.S. Truscott, Triboelectrification of identical insulators, Journal of Physics D 19 (2000) 1273e1298. [20] D.R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, 2012. [21] J. Lowell, Bidirectional charge transfer in repeated contacts to polymers, Journal of Electrostatics 20 (1987) 233e238.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Generation of hard X-ray radiation using the triboelectric effect by peeling adhesive tape Hartmut Stöcker a, *, Maximilian Rühl b, Anett Heinrich a, Erik Mehner a, Dirk C. Meyer a a b
TU Bergakademie Freiberg, Institut für Experimentelle Physik, Leipziger Str. 23, 09596 Freiberg, Germany TU Dresden, Institut für Fertigungstechnik, George-Bähr-Str. 3c, 01069 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2013 Received in revised form 3 July 2013 Accepted 12 July 2013 Available online 24 July 2013
The triboelectric effect describes electrical charging when bringing different materials into contact. We report on the generation of hard X-ray radiation by peeling various adhesive tapes under medium vacuum conditions. Beside vacuum housing and pumps as instrumentation only an electric motor, two rolls and a metal foil as target material are necessary. The spectral distribution of generated X-rays was analyzed using an energy-dispersive detector. Depending on peeling speed, pressure and choice of material combination, electrons with energies sufficient to excite emission in the hard X-ray region are produced. The results are discussed in terms of triboelectric separation of charge carriers. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: X-ray generation Triboelectric effect Adhesive tape
1. Introduction To meet the demands of medical imaging as well as scientific methods for composition and structure investigations, generation of X-rays using miniaturized radiation sources is an interesting field of research. Beside approaches based on the ionizing and electron accelerating properties of high electric fields around pyroelectric crystals [1] also the use of a tribomicroplasma generated in the vicinity of the peeling point of two different polymers is promising [2e5]. The miniaturization of an appropriate setup could be used to engineer portable X-ray sources for the above applications. The triboelectric effect describes an electrical charging of different materials brought into contact [6]. The polarity and strength of the charges depend on the materials chosen and their surface roughnesses. Polymers are typical examples of materials that can acquire such charging [7,8]. If a high density of surface states is available and the work functions FA and FB of the two materials differ, contacting them leads to a band bending (see Fig. 1). Thus, electrons and possibly also ions [9] are transferred across the interface and charges build up on the respective polymers. Sign and amount of charging are described empirically by the so-called triboelectric series [10]. Thanks to this effect, peeling of the two polymers leads to an electric voltage and to the generation
* Corresponding author. Tel.: þ49 3731392773; fax: þ49 3731394314. E-mail address: [email protected] (H. Stöcker). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.07.006
of a plasma at the peeling point [11,12]. Hence, charged carriers are accelerated in the electric field within the gap. If these electrons or ions hit a target, X-rays and other forms of electromagnetic radiation are generated [13e17]. The present study aims to find an optimum set of parameters for X-ray production by peeling adhesive tape. Therefore, different materials and types of contacts will be tested and peeling speed as well as vacuum pressure varied. 2. Materials and methods The setup consists of a vacuum chamber made of glass and stainless steel that houses two spools, with one driven by an external motor with variable speed and the other one rotating freely. A turbo molecular pump with membrane pre-pump evacuates the vacuum chamber to pressures between 102 and 104 mbar. A titanium foil acting as electron target to produce characteristic X-rays is fixed underneath the rolls holding the adhesive tape. An energy-dispersive silicon drift detector (Röntec, Berlin) is connected to the chamber through a thin Kapton window for registering the produced X-rays with minimal absorption losses. Within the chamber, the adhesive tape can be fixed in two ways: Firstly, a full roll of tape is mounted on the freely rotating spool and peeled off by reeling it on the other spool using the external motor. This has the disadvantage of measurement time being limited by the length of the tape. Therefore, secondly, a closed piece of tape is connected around both spools in such a way that the adhesive tape
906
H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 1. Electronic band scheme at the interface of two polymers A and B before (a) and after contact (b). Work functions F and Fermi energies EF as well as occupied surface states are indicated. When a contact is established, the high density of surface states leads to band bending and a potential difference.
is running infinitely around the spools when starting the motor (see Fig. 2). This latter setup was used for the present investigations. Several types of commercially available adhesive tape have been used. Details and sample labels are given in Table 1. Apart from X-ray generation tests, surface energy and adhesion force measurements were performed. Surface energies were evaluated from contact angles measured by a Dataphysics OCA 15 EC. Adhesion forces were determined by tangential pulling using a spring scale. 3. Results 3.1. Surface energies and adhesion forces Surface energies were calculated from measured contact angles so that separation of disperse and polar contributions was possible. The results for the samples are given in Fig. 3. The disperse contributions are relatively small, especially for the adhesive sides of the tape. The latter point is apparently desirable for good adhesion of the tape. Comparing the polar contributions of carrier and adhesive side, sample Te1 shows only a small difference whereas the other three samples exhibit bigger and very similar differences. The adhesion forces between carrier and adhesive material measured by a spring scale are given in Table 2 for the four samples investigated before. Sample Te1 exhibits the highest adhesion and Sco1 the lowest. A direct relation with the surface energies is not obvious; however, the electrostatic part of the adhesion force is correlated with the polar part of the surface energy (see Discussion). 3.2. Triboelectric generation of X-rays The X-ray spectra obtained by peeling adhesive tape in vacuum feature a broad background of Bremsstrahlung and several characteristic X-ray emission lines (see Fig. 4). The most intense line is of course related to the titanium foil that was fixed as an electron target underneath the adhesive tape (see Fig. 2). Further lines are related to the components of the vacuum chamber (stainless steel, O-rings etc.) that are as well excited to produce X-ray fluorescence. This was verified by measurements without the Ti foil.
Comparing the spectra obtained for different peeling velocities (see Fig. 5), sample Sco2 exhibits a clear intensity maximum for an intermediate velocity of 10 cm/s. The maximum obtainable X-ray energy was found to be 79 keV at a pressure of 103 mbar and a velocity of 10 cm/s. The maximum X-ray intensity is found in the energy range between 2 and 10 keV. Plotting the X-ray count rate over time reveals quite different curves depending on peeling velocity (see Fig. 6). Nevertheless, all curves show a negative slope, i.e. a steady decrease of X-ray intensity over time is observed. This is mainly related to the degradation and detaching of the adhesive polymer. The longest time of continuous X-ray generation for sample Sco2 was measured as 55,710 s (approx. 15.5 h) at a pressure of 103 mbar and a velocity of 8 cm/s. For all of the samples listed in Table 1, X-ray generation was tested for different peeling velocities and vacuum pressures. The three samples with the strongest X-ray output are compared in Fig. 7. The two Scotch samples are delivering the highest X-ray intensity. A maximum integral intensity of 2.8 million counts was obtained with material Sco2 at a pressure of 103 mbar and a velocity of 10 cm/s, which relates to a count rate of 519 cps. Decreasing the pressure to 104 mbar provides slightly lower optimum intensities but increases the X-ray output at lower velocities. 3.3. Dependence on the type of contact In addition to studying adhesive-carrier contacts (see previous section), also contacts with metals were investigated. The contact between adhesive side of the tape and the different metals was established by using metal foils on the spools or the spool itself that is made of stainless steel. For contacting the carrier side with a metal, an additional free spool was installed within the vacuum chamber and pressed against a roll of tape on the existing driven spool. This setup, employing an additional spool pressing against the driven spool allowed to study also contacts between two adhesive sides as well as between two carrier sides of adhesive tape. Nevertheless, no significant X-ray output was found for these two combinations. For the polymeremetal contacts, it is found that for all three metals, the contact with the carrier side of the tape provides higher X-ray intensities than with the adhesive side (see Fig. 8). Comparing the metals, aluminium clearly provides the highest output showing additionally a slight intensity increase with rising peeling velocity. Finally, copper provides slightly higher X-ray intensity than steel, depending on peeling speed. Table 1 Sample list.
Fig. 2. Schematic setup for triboelectric X-ray generation by peeling of adhesive tape.
Sample
Manufacturer
Colour
Material
Width
Pri1 Sco1 Sco2 Te1
Printus Scotch Scotch Tesa
Transparent Transparent Milky Transparent
Polypropylene Polypropylene Cellulose acetate Polypropylene
19 19 19 15
mm mm mm mm
H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 3. Measured surface energies for the four samples divided in carrier side (C) and adhesive side (A) of the tape.
4. Discussion Using DLVO theory [18] and regarding the peeling point as plate capacitor with dielectric leads to a description for the adhesion force FA as sum of van der Waals force FW and electrostatic force FE: FA ¼ FW þ FE. The electrostatic force can be evaluated from the measured quantities by FE ¼ gp/gtot$FA where gp is the polar part of the surface energy and gtot the total surface energy. Additionally, for a plate capacitor FE ¼ Q$E ¼ Q$U/d is applicable using charge Q, electric field E, voltage U and plate distance d. Using the equation E ¼ Q/(ε0εrA) for the electric field with the dielectric constant ε0 and the sample permittivity εr, finally leads to an expression for the expected voltage:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gp F A d 2 U ¼ gtot ε0 εr A
(1)
Obviously, the assumption of the peeling point being a parallel plate capacitor is a drastic simplification and, additionally, the determination of A and d for any given setup is not straightforward. Nevertheless, this approximation gives a good idea of the relation between adhesion and triboelectricity and how to optimize the material properties for maximum electron acceleration voltage. Namely, a high ratio gp/gtot and a high adhesion force FA are advantageous. Unfortunately, this derivation is only valid under certain circumstances and not in accordance with experimental observations. As shown in the Results section, adhesion forces above 3 N (samples Pri1 and Te1) lead to a reduced X-ray production. This is explainable because a high adhesion force leads to detaching of the adhesive from the carrier polymer so that the X-ray generation
907
Fig. 4. Assignment of characteristic X-ray emission lines in the generated spectrum for sample Sco2 at a velocity of 12 cm/s and a pressure of 103 mbar accumulated for 90 min.
decreases quickly after starting the experiment. The model used for Eq. (1) neglects this unwanted delamination mechanism so that simply maximizing the adhesion force is finally not the ideal choice for triboelectric X-ray generation. Regarding the surface energies, the experiments suggest that a high difference between the surface energies of carrier and adhesive side of the tape leads to the highest X-ray output. Since a high polar contribution of surface energy is favourable for generation of electric fields, the comparison can also be carried out using gp alone. Then, it is found that a difference of approx. 9 mN/m between carrier and adhesive polymer is optimal for our specific setup. Discussing the different types of contacts, it is helpful to rely on the simple model of Fig. 1. It is obvious that no potential is formed between two equal polymers. In accordance with that, no X-ray intensity was observed when contacting equal polymers (adhesiveeadhesive and carrierecarrier). Nevertheless, only small deviations of the electronic surface state occupation are necessary to produce charging between polymers [19]. Replacing one polymer by a metal can result in remarkable Xray output. The difference between the emissions of the metal contacts is explainable simply by their work functions. Contacting a polymer with a metal, a situation similar to the scheme given in Fig. 1 results with the metal being material A (left side). Since the surface states of the polymer adjust to the Fermi level of the metal,
Table 2 Adhesion forces measured between carrier and adhesive side of the tape. Sample
Adhesion force (N)
Pri1 Sco1 Sco2 Te1
3.1 0.9 1.8 3.8
0.2 0.2 0.2 0.2
Fig. 5. Dependence of X-ray emission spectrum on the peeling velocity for sample Sco2 at a pressure of 103 mbar accumulated for 90 min.
908
H. Stöcker et al. / Journal of Electrostatics 71 (2013) 905e909
Fig. 6. Dependence of total X-ray count rate on time and peeling velocity for sample Sco2 at a pressure of 103 mbar.
intensity than copper or steel with work functions of 4.6 eV and 4.7 eV, respectively [20]. It has to be noted that the highest occupied surface state of the polymer may change between subsequent contacts [21], so that an uncertainty range for the work function has to be taken into account. Also, the generated X-ray intensity is modified by the different X-ray yields of the elements acting as target material. Nevertheless, for our setup with additional Ti target foil producing most of the X-ray output, this effect is greatly suppressed. Since the X-ray output depends on the difference between work function of the metal and highest occupied surface state in the polymer, a method for determining this state can be proposed: A metalepolymer contact that does not produce X-rays during peeling relates to a flat band situation at the interface and, hence, the highest occupied surface state of the polymer is equal to the (known) work function of the metal. From our experiments, we can therefore conclude a highest occupied surface state in the polymer at approx. 5 eV below the vacuum level. Since the X-ray output of polymeremetal contacts is always higher for the carrier side, we find that the level of the highest occupied surface state for the adhesive side is above this level at the carrier side. 5. Conclusions
Fig. 7. Comparison of total X-ray counts for different materials, vacuum pressures and peeling velocities during an accumulation period of 90 min. The intensity errors were calculated from 3 consecutive runs.
the work function of the metal defines the potential difference at the interface. For low work function FA, a high potential results and vice versa. Therefore, aluminium with a work function of approx. 4.1 eV produces a higher potential and a higher X-ray
We have shown that the generation of X-rays using adhesive tape works best if the difference between the polar part of the surface energies of the two polymers is in the range of 9 mN/m for the described setup. Additionally, if the adhesion force crosses a maximum of 3 N, the X-ray generation is suppressed by detaching of the adhesive from the carrier polymer. For sample Sco2 at a pressure of 103 mbar and a velocity of 10 cm/s the highest photon count rate of approx. 500 counts per second was observed. Also metalepolymer contacts were found to deliver X-ray output depending on the work function of the metal. This observation could possibly be used to determine the highest occupied surface state of the polymer. Appendix. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.elstat.2013.07.006. References
Fig. 8. Comparison of total X-ray counts for three different metals in contact with carrier or adhesive side of sample Sco1 measured in dependence on peeling velocity at a vacuum pressure of 102 mbar during an accumulation period of 40 s.
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