- Email: [email protected]
Radiation-induced changes in the electrical properties of carbon filled PVDF thick films
Materials Science and Engineering B 141 (2007) 115–120
Radiation-induced changes in the electrical properties of carbon filled PVDF thick films O. Korostynska a , K. Arshak a,∗ , D. Morris b , A. Arshak b , E. Jafer a a
Electronic & Computer Engineering Department, University of Limerick, Limerick, Ireland b Physics Department, University of Limerick, Limerick, Ireland Received 15 March 2006; received in revised form 24 June 2007; accepted 30 June 2007
Abstract The electrical properties of PVDF thick film capacitors under gamma radiation are investigated. To increase the conductivity of the films, they were filled with 4 and 6 wt.% of carbon, which is close to the percolation threshold. Screen-printing was used for film fabrication. All films were exposed to a disk-type 137 Cs source with an activity of 370 kBq. Changes in I–V characteristics were measured after each exposure dose. A tenfold increase in the values of current was recorded after a dose of 228 Gy for C-PVDF films with a thickness of 23.97 m and 6 wt.% carbon doping. A higher dose of 342 Gy resulted a decrease in the values of current. Thicker films showed an increase in the values of current with irradiation to a dose of 798 Gy. PVDF + carbon system has potential applications in low-dose radiation dosimetry. The high current induced by radiation caused heating and electroforming of the device, due to the metal inclusions from the Ag contact material. It was noticed that as-printed films of 23.97 m in thickness, tend to electroform at about 12 V, whereas films irradiated with 171 Gy showed a strong electroforming effect at a lower voltage of 5 V. For that reason, proper design of dosimetry systems is essential to eliminate such effects. Gamma radiation sensitivity of counterpart PVDF thick films with 4 wt.% carbon doping was studied via capacitance-dose measurements in real time, since these samples were less conductive. Irradiation of this sensor with doses from 1.15 to 2.5 mGy caused a considerable monotonic increase in the values of its capacitance from 2.92 to 12.37 pF. Accordingly, sensors with 4 wt.% of carbon could sustain higher radiation doses, but had poor sensitivity to radiation of lower level. © 2007 Elsevier B.V. All rights reserved. Keywords: Gamma radiation; Thick film; PVDF; Carbon; Electrical properties
1. Introduction Adequate personal radiation dosimetry is highly essential in a wide range of areas, for example, medicine, industry, security and science. Passive dosimeters, such as photographic films and thermoluminescent materials, are largely used for individual dosimetry of ionization radiation, but they do not provide a direct reading and need a further treatment after irradiation to yield any information on the dose level [1]. Moreover, there is a need for compact, affordable, real-time active dosimeters, with a visual/audio alarm, if a critical threshold radiation dose is exceeded. Metal oxides and polymers are regarded as candidate materials for radiation sensing layers [1–3]. The advances in processing of conducting polymers have contributed to the development of new types of radiation detectors, which can be
∗
Corresponding author. Tel.: +353 61202267. E-mail address: [email protected] (K. Arshak).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.06.025
used for in situ measurements. Deep understanding of physical properties of the materials under the influence of radiation is vital for the effective design of dosimeters [4,2]. High-energy radiations, such as ␥-rays change the physical properties of the materials they interact with. The changes are strongly dependent on the internal structure of the material, on dose and on the parameters of the films including their thickness. The degradation is more severe for higher doses and thinner films [5,6]. Radiation interacts with polymers in two ways: chain scission, which results in reduced tensile strength and elongation; and crosslinking, which increases tensile strength but reduces elongation. Both reactions occur simultaneously, but one is usually predominant, depending upon the specific polymer and additives involved. A linear correlation of polymer conductivity with the applied dose was reported [3,1]. Two non-conducting forms of polyaniline were used as a detecting device for low-dose ionization radiation, whereas the conducting form manifested itself as a possible high dose detector [1]. The radiation interaction was mainly due to an oxidation process of the main polymer
116
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
chain yielding a conductivity alteration. Ionising radiation also induced a doping state in the polymer main chain structure similar to that found in the conventional acid doping process [1]. Polyaniline/poly acid acrylic composite thin films were used as ionization radiation sensors. Preliminary gamma radiation interaction with composite showed a linear response that can be used in the development of real-time radiation sensors for dose range from 0 to 5000 Gy [1]. Polyaniline optical dosimeter for 60 Co gamma radiation was based on colour change of 40 nm thick nanofilms [7]. The deep blue colour, which is a characteristic of undoped polyaniline film before irradiation, became subsequently green as the film was irradiated to a dose range from 0 to 10 kGy. Polyvinylidene fluoride (PVDF) is a long chain highmolecular-weight polymer with predominant repeating unit established as (–CH2 –CF2 –). This semi-crystalline material with a melting point of 338 ◦ C is widely used for various technical/engineering applications. Several types of sensors based on PVDF film were designed and tested, including laser radiation sensors, a light pressure transducer, an ion radiation sensor, and a multifunctional alarm [8]. Pure and doped PVDF films were employed in a pyroelectric detector measuring short laser pulses in the sub-nanosecond range [9]. Semi-crystalline PVDF, irradiated with different ions in the range 100 keV–100 MeV, exhibited a strong change in the crystalline structure, as detected by X-ray diffraction measurements and calorimetric analysis [10]. The relationship between the crystallinity of PVDF and electron radiation effects was studied with differential scanning calorimetry and X-ray diffraction [11]. It was reported that at doses lower than 400 kGy, the crystallinity of PVDF increased slightly with the increase in absorbed dose, and then decreased slowly. A decrease in the crystallinity was accelerated by the increase in the absorbed dose at higher dose zone [11]. At ion doses higher than 300 kGy, PVDF showed a degradation in the crystalline structure, reduction of hydrogen and fluorine concentration and the formation of carbon reticulation [10]. This work focuses on the development of low-dose gamma radiation sensor based on screen-printed PVDF thick films. To enhance the conductivity of the overall mixture, carbon was added into PVDF, at the amount close to percolation threshold. Gamma radiation-induced changes in the electrical properties of PVDF capacitors were the bases for dose estimation. Conducting composites based on conductive particles in a host matrix are widely used owing to their unique electrical and mechanical properties [12]. Most polymer thick films are typical insulators and the conductivity of the composite materials is predominantly dependant on the content and properties of the filler as well as on composite structure. New properties may originate from specific interaction between the material and the added amount of carbon in addition to the particular arrangement of the filler particles in the host matrix [13]. 2. Experimental procedure The main ingredient of the paste was PVDF powder, supplied by Sigma Aldrich. To improve the conductivity of thick films,
Fig. 1. Schematic structure of capacitor structure, where 1 is alumina substrate; 2 is Ag electrodes and 3 is carbon-filled PVDF thick film.
6 wt.% of carbon black powder was also added into the paste. It was shown that pure PVDF is tolerant to radiation of up to ∼300 kGy. PVDF used in medicine has radiation tolerance level of around 1000 kGy. In our experiments we used Carbon doping to make devices sensitive to much lower doses of gamma radiation. In practise, pure PVDF devices with the same configuration were too insulative, with no current passing through the circuit under the applied voltage of up to 10 V. Higher voltages are not feasible. Contrary, pure carbon paste is too conductive. Carbon black was supplied by Sigma Aldrich and had a particle size of 42 nm. This was added to 7 wt.% ethyl cellulose and 1 wt.% of surfactant (supplied by Lennox Ltd) to surfactant to achieve a homogeneous dispersion of carbon particles. The surfactant was a polymeric amphiphile composed of anchoring groups, which are adsorbed onto the surface of the carbon black particles. The chemical structures of the hypermer dispersing agents and also their large molecular weights provide a steric barrier to minimise particle–particle interaction and thus reduce re-agglomeration of carbon black [14]. Finally, Terpinol-␣ (Sigma Aldrich) was used as the solvent. The samples were fabricated by screen-printing on alumina substrates. Active area of capacitive structures measured 2 mm × 3 mm, while Ag paste was chosen for electrodes material (see Fig. 1). DEK RS 1202 automatic screen printer was used for device fabrication. After carbon-filled PVDF paste was printed onto bottom electrode, it was dried in air and cured at 100 ◦ C for 30 min and then at a peak temperature of 170 ◦ C for 10 min. In-house printing of resistors using thick-film technology can result in resistor tolerances of ±20%. At industrial manufacturing conditions considerably higher sample reproducibility can be achieved. When using screen-printing for thick film manufacture, there is a limit of how thin the layer can be. It depends on the material itself, its particle size, on the mesh, through which the paste is squeezed during printing, on the number of screen-prints, and on the firing conditions (normally layers are thinner after drying). At least few layers of carbon-filled PVDF paste were required to ensure repeatability of the devices and, more importantly, to avoid possible pinhole formation, which could lead to the shortage of the device when electrodes are applied. A number of samples with two different thicknesses were manufactured. A Sloan Dektak 900051 Profilometer was used to determine the average thickness of the dielectric layers, which was found to be 23.97 and 39.12 m for samples #1 and #2,
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
117
capacitive interface circuitry. The system consists of on-chip temperature sensor, 24-bit SD modulator, digital filter, voltage regulator and serial interface—all integrated in one module. The module can operate with a single power supply of 2.7 V. The output information from the digital filter is read by the microcontroller unit through a serial interface part. Capacitive sensors exhibit a change in capacitance in response to a change in physical stimulus. Most of the designed capacitive systems are based on converting the capacitance to voltage first. Then this voltage is converted into digital domain with high precision analog-todigital converter (ADC). Detailed information on the wireless capacitive module can be found elsewhere [15]. 3. Results and discussion
Fig. 2. The picture of C-PVDF thick film surface.
respectively. A surface roughness of both films was about 1 m, therefore thick film Ag electrodes were used. Fig. 2 shows the picture of C-PVDF thick film surface, which was taken using Olympus Microscope BX60. To investigate the effect of radiation on the crystalline structure of the material, qualitative X-ray powder diffraction (XRD) was performed using a Philips X’pert PRO multi purpose diffractometer (MPD) X-ray diffractometer PW3050/60 θ–θ (Philips, Eindhoven, Netherlands) within a scan range of 5–70◦ (2θ). The CuK␣ diffractometer anode was run under a tension of 40 kV and a current of 35 mA. An X’Celerator strip detector was used to collect the diffracted data. A glancing angle of 3◦ (2θ) was applied in order to prevent anomalous data being obtained from the glass slide. 137 Cs (0.662 MeV) disk-type source with an activity of 370 kBq (provided by AEA Technology QSA GmbH as a standard reference gamma radiation source) was used to expose the samples to ␥-radiation. The radioactive gamma-emitting element (3.18 mm × 5 mm) was encapsulated into a 2 mm thick high strength epoxy resin (diameter 25 mm) to shield any accompanying -radiation. The source was held at a distance of 1 cm from the device under investigation at an angle of incidence of 0◦ . A set of irradiations were performed, changing the exposure time and hence the dose. Values of radiation damage were estimated from changes in the current–voltage characteristics of the C-PVDF capacitors. The I–V measurements were taken after each irradiation. To investigate the effect of carbon loading on the radiation sensitivity of the PVDF thick films, device having identical structure to samples #1 and #2, but 4 wt.% of Carbon, was tested. This is denoted as sample #3. Reduced carbon loading resulted a decreased conductivity of thick film sensor. Therefore, capacitance versus dose measurements were used to trace the effect of gamma radiation on the sample #3 electrical properties. The changes in the values of capacitance with radiation were measured wirelessly in real-time via specially designed
PVDF is a semicrystalline ferroelectric polymer, which can crystallise into four different crystal forms: ␣, , ␥, and ␦ depending upon the crystallisation conditions. The most common form is the ␣-phase, which is non-polar. The ␣-phase can be transformed to highly polar -phase by mechanical deformation and pressure [16]. Fig. 3 depicts the XRD scan for PVDF thick film on glass substrate. This spectrum indicates the presence of semi-crystalline structure. As expected, the film is predominantly ␣-phase. The main peaks, with their ‘h k l’ values in brackets, associated with ␣-phase are at 2θ values of 17.9◦ [1 0 0], 18.4◦ [0 2 0], 20.1◦ [1 1 0] and 26.7◦ [0 2 1] [17,18]. No phase transition was recorded due to the effect of radiation dose of ∼10 mGy, e.g. the XRD scan remained identical to Fig. 3. The electrical properties of mixtures containing carbon black are widely explained and experimentally supported within the framework of inter-particle tunnelling conduction and/or the framework of classical percolation theory [19–21]. Percolation theory considers, in the simplest case, the formation of clusters in the volume of a matrix. Those clusters, after reaching an appropriate concentration of filler particles, could form an infinite network. In the case of conducting filler particles, the conductivity in the system should increase dramatically after reaching the condition, known as the percolation threshold [20]. Basic models, which consider the dispersion of spherical particles throughout an insulating matrix, show that the conductivity (σ) of the system varies so that σ = σ 0 (ν − νc )t , where ν is the volume fraction of conductive filler and νc is the critical volume fraction, which occurs at the percolation threshold. This important point coincides with the formation of the first continuous,
Fig. 3. XRD patterns of carbon-filled PVDF thick film on a glass substrate.
118
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
Fig. 4. Changes in current–voltage plots with dose for C-PVDF sample #1. Fig. 5. Changes in current–voltage plots with dose for C-PVDF sample #2.
conducting pathway, which spans the entire system. For volume fractions above the percolation threshold, the conductivity of the composite is closer to that of the filler, which indications that an unbroken conduction networks exist [12]. The exponent t is assumed to be universal and conveys the changing conductivity with conducting filler concentration, while σ 0 is the conductivity of the fillers or conductive phase. From the above description, it may be understood that the percolation threshold corresponds to an increase in conductivity of many orders of magnitude and so the sensitivity of the entire system is highly dependant on the concentration of conducting particles present [12]. The electrical conductivity of carbon black containing composites was found to depend on the structure of the carbon black particles and to deviate from the expectations of classical percolation theory [19]. In this study, reproducible samples with a given fraction of carbon black, prepared under the same processing conditions have been achieved. Hence, a very similar spatial distribution of carbon black should be expected. Generally, a trend can be noticed that the larger the inhomogeneity in carbon black distribution, the smaller is the effective conductivity of the composite. This is why mixing processes require proper monitoring, e.g. automated shear mixing is recommended. Fig. 4 displays changes in current–voltage plots with dose for PVDF sample #1, i.e. having a thickness of 23.97 m. One may see that a monotonic tenfold increase in the values of current was caused by irradiation with gamma dose of 228 Gy. A higher dose of 342 Gy was critical to the performance of these devices and a considerable decrease in the values of current was recorded. Therefore a range of 0–228 Gy can be considered as the working dose range for these carbon-filled PVDF capacitors. Annealing of #1 thick films for 4 h in a conventional oven at 80 ◦ C was found to fully restore their original properties after the irradiation. This suggests that these films can be reused. However, if the films are subjected to much higher doses of radiation, some permanent defects can occur. To remove them, elevated temperatures are needed, which in turn can change the structure and properties of the polymer thick film. Fig. 5 displays changes in current–voltage plots with dose for a thicker sample #2. These films showed an increase in the values of current with increasing gamma dose up to a level of 798 Gy. This level is higher than 228 Gy of the sample #1.
Unlike sample #1, there was no rapid decrease in the values of current recorded for the sample #2, when it was irradiated with a dose higher than 798 Gy. Instead, a slow monotonic decrease was monitored. To trace and compare the performance of these two samples with different thicknesses, see Fig. 6, which shows the dependence of normalised currents (I − I0 )/I0 with radiation dose for both samples at the same fixed applied voltage of 3 V. It is important to mention that at higher applied voltages, the effect of gamma radiation on the electrical properties of carbonfilled PVDF thick film capacitors was more pronounced, e.g. the changes in the values of current were bigger. However, highapplied voltage causes additional Joule heating, leading to an increased migration of conducting particles through the polymer matrix. Moreover, Joule heating caused metal inclusions from the Ag electrode material into the polymer sensing layer. As a result, an electroforming effect was recorded at 12 V for asprinted PVDF thick film #1, whereas films irradiated with a dose of 171 Gy showed strong electroforming effect already at a lower voltage of 5 V. Accordingly, after electroforming takes place, the device is no longer considered reliable for dosimetry purposes. Fig. 7 illustrates radiation-induced changes in the value of capacitance of C-PVDF sample #3, e.g. with 4 wt.% carbon doping. Irradiation of this sensor with doses from 1.15 to 2.5 mGy
Fig. 6. Dependence of normalised currents (I − I0 )/I0 with radiation dose at a fixed voltage of 3 V for samples #1 and #2.
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
Fig. 7. Changes in the value of capacitance of C-PVDF sample #3 with radiation.
caused a considerable increase in the values of its capacitance from 2.92 to 12.37 pF. The value of 1.15 mGy is regarded here as minimum detectable dose (MDD), since no measurable changes with radiation were recorded prior to that [22]. However, MDD is affected by material composition and device structure, including thickness. After a dose of 2.5 mGy C-PVDF sensor with 4 wt.% carbon showed saturating response, therefore a dose range of 1.15–2.5 mGy is considered as a working dose range for this particular radiation sensor. Generally, the ␥-radiation-induced changes in the electrical characteristics of the samples were found to be similar to the dose response of most materials used in thermoluminescence (TL) dosimetry [23]. They usually show a linear, then supralinear, and finally a saturating response, beyond which damage occurs with the increase in dose. The details of the experimentally measured dose response depend on a large number of parameters, related to the instruments, radiation field and fabrication process [23]. Differences in material properties could determine the nature of changes in the supralinear region on the electrical parameters, under radiation exposure. However, it is the linear response region, which is preferable in radiation dosimetry. All these effects should be taken into consideration, when designing a radiation sensing system, as one has to compromise between a higher response resolution and wider working dose range. 4. Conclusion The aim of this work is to develop cost-effective sensors for personnel dosimetry application. Each device has its limitation, e.g. max applied voltage, max/min operating temperature, load and so forth. For the devices under investigation, one of the most important parameters is the working dose range, beyond which the device is considered as damaged. Device response is correlated with the real applied dose only in the working dose range of that particular device. Unfortunately, no correlation is found after these devices have been damaged. The electrical properties of PVDF based screen-printed thick films were explored in terms of gamma radiation influence. A polymer paste was filled with 4 and 6 wt.% of carbon black
119
to increase the conductivity of the device. Moreover, increased conductivity resulted in enhanced sensitivity of the PVDF thick films to ␥-rays. A monotonic tenfold increase in the values of current was recorded for samples #1 having thickness of 23.97 m and 6 wt.% of carbon, after the irradiation with a dose of 228 Gy. Annealing of these samples for 4 h in a conventional oven restored their properties after they were irradiated. This indicates the possibility to reuse the films. Thicker PVDF films (sample #2) could sustain higher radiation dose of 798 Gy and slow decrease in the values of current was monitored with further increase in dose. Gamma radiation sensitivity of counterpart PVDF thick films with 4 wt.% carbon doping was studied via capacitance-dose measurements in real time. Irradiation of this sensor with doses from 1.15 to 2.5 mGy caused a considerable monotonic increase in the values of its capacitance from 2.92 to 12.37 pF, whereas the value of 1.15 mGy is regarded as minimum detectable dose. Combining sensors with various levels if carbon doping into sensor array would enhance radiation sensitivity of the overall system. Based on the experimental results, the manufacture of polymer based thick film structures should be considered as a cost-effective alternative to traditional wafer-based fabrication techniques, as it can produce sensors with real time dose measurement of ␥-radiation at room temperature. Acknowledgements Funding was received from the Irish Research Council for Science, Engineering and Technology: funded by the National Development Plan. References [1] A.P. Lima Pacheco, E.S. Araujo, W.M. de Azevedo, Mater. Character. 245 (2003) 50. [2] K. Arshak, O. Korostynska, Mater. Sci. Eng. B 224 (2004) 107. [3] K. Arshak, A. Arshak, S. Zleetni, O. Korostynska, Nucl. Sci., IEEE Trans. 2250 (2004) 51. [4] K. Arshak, O. Korostynska, F. Fahim, Sensors 176 (2003) 3. [5] K. Arshak, O. Korostynska, IEEE Sens. 717 (2003) 3. [6] E. Atanassova, A. Paskaleva, R. Konakova, D. Spassov, V.F. Mitin, Microelectron. J. 553 (2001) 32. [7] J.M.G. Laranjeira, H.J. Khoury, W.M. de Azevedo, E.A. de Vasconcelos, J. da Silva, Phys. E: Low-Dimensional Syst. Nanostruct. 666 (2003) 17. [8] W. Shuduo, Polyvinylidene fluoride film sensors and applications, in: Proceedings, Berlin, Germany, 1992, pp. 923–928. [9] B. Elling, W. Kuenstler, R. Danz, D. Geiss, W. Bohmeyer, Detection of laser radiation using PVDF-films, in: Proceedings of the 6th International Symposium Electrets, 1988, pp. 399–403. [10] L. Torrisi, G. Ciavola, G. Foti, R. Percolla, Nucl. Instrum. Methods Phys. Res., Sect. A: Accel., Spectrom., Detectors Assoc. Equip. 361 (1996) 382. [11] S. Hongbing, X. Liqing, C. Jun, W. Haiyan, Sci. China, Series A: Math., Phys., Astron. 438 (1998) 41. [12] J. Vilc´akov´a, P. S´aha, O. Quadrat, Eur. Polym. J. 2343 (2002) 38. [13] J. Barkauskas, A. Vinslovaite, Mater. Res. Bull. 1437 (2003) 38. [14] K. Arshak, E. Moore, L. Cavanagh, J. Harris, B. McConigly, C. Cunniffe, G. Lyons, S. Clifford, Compos. Part A: Appl. Sci. Manuf. 487 (2005) 36. [15] K. Arshak, O. Korostynska, E. Jafer, A. Arshak, D. Morris, E. Gill, Gamma Radiation Sensing Using ZnO and SnO2 Thick Film Interdigitated Capac-
120
[16]
[17] [18] [19]
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120 itors, in: Proceeding of the IEEE ISSE, Dresden, Germany, 2006, pp. 495–500. K. Arshak, C. Ryan, M. Campion, Pressure induced piezoelectric/spl beta/phase crystallization in screen printed polymer thick films, in Proceedings, 2002, pp. 411–414. A. Tawansi, A.H. Oraby, S.I. Badr, I.S. Elashmawi, Polym. Int. 370 (2004) 53. L. Priya, J.P. Jog, J. Appl. Polym. Sci. 2036 (2003) 89. I. Balberg, Carbon 139 (2002) 40.
[20] G. Grimmett, Percolation, second ed., Springer Verlag, Berlin, 1999. [21] A. Dziedzic, A.A. Snarskii, B.W. Licznerski, Some remarks about temperature dependence of resistance in percolation system, in: Proceedings of the SPIE Poland, vol. 2780, 1995, pp. 95–99. [22] K. Arshak, O. Korostynska, Advanced Materials and Techniques for Radiation Dosimetry, Artech House, Boston, 2006. [23] Y.S. Horowitz, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 68 (2001) 184.
Radiation-induced changes in the electrical properties of carbon filled PVDF thick films O. Korostynska a , K. Arshak a,∗ , D. Morris b , A. Arshak b , E. Jafer a a
Electronic & Computer Engineering Department, University of Limerick, Limerick, Ireland b Physics Department, University of Limerick, Limerick, Ireland Received 15 March 2006; received in revised form 24 June 2007; accepted 30 June 2007
Abstract The electrical properties of PVDF thick film capacitors under gamma radiation are investigated. To increase the conductivity of the films, they were filled with 4 and 6 wt.% of carbon, which is close to the percolation threshold. Screen-printing was used for film fabrication. All films were exposed to a disk-type 137 Cs source with an activity of 370 kBq. Changes in I–V characteristics were measured after each exposure dose. A tenfold increase in the values of current was recorded after a dose of 228 Gy for C-PVDF films with a thickness of 23.97 m and 6 wt.% carbon doping. A higher dose of 342 Gy resulted a decrease in the values of current. Thicker films showed an increase in the values of current with irradiation to a dose of 798 Gy. PVDF + carbon system has potential applications in low-dose radiation dosimetry. The high current induced by radiation caused heating and electroforming of the device, due to the metal inclusions from the Ag contact material. It was noticed that as-printed films of 23.97 m in thickness, tend to electroform at about 12 V, whereas films irradiated with 171 Gy showed a strong electroforming effect at a lower voltage of 5 V. For that reason, proper design of dosimetry systems is essential to eliminate such effects. Gamma radiation sensitivity of counterpart PVDF thick films with 4 wt.% carbon doping was studied via capacitance-dose measurements in real time, since these samples were less conductive. Irradiation of this sensor with doses from 1.15 to 2.5 mGy caused a considerable monotonic increase in the values of its capacitance from 2.92 to 12.37 pF. Accordingly, sensors with 4 wt.% of carbon could sustain higher radiation doses, but had poor sensitivity to radiation of lower level. © 2007 Elsevier B.V. All rights reserved. Keywords: Gamma radiation; Thick film; PVDF; Carbon; Electrical properties
1. Introduction Adequate personal radiation dosimetry is highly essential in a wide range of areas, for example, medicine, industry, security and science. Passive dosimeters, such as photographic films and thermoluminescent materials, are largely used for individual dosimetry of ionization radiation, but they do not provide a direct reading and need a further treatment after irradiation to yield any information on the dose level [1]. Moreover, there is a need for compact, affordable, real-time active dosimeters, with a visual/audio alarm, if a critical threshold radiation dose is exceeded. Metal oxides and polymers are regarded as candidate materials for radiation sensing layers [1–3]. The advances in processing of conducting polymers have contributed to the development of new types of radiation detectors, which can be
∗
Corresponding author. Tel.: +353 61202267. E-mail address: [email protected] (K. Arshak).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.06.025
used for in situ measurements. Deep understanding of physical properties of the materials under the influence of radiation is vital for the effective design of dosimeters [4,2]. High-energy radiations, such as ␥-rays change the physical properties of the materials they interact with. The changes are strongly dependent on the internal structure of the material, on dose and on the parameters of the films including their thickness. The degradation is more severe for higher doses and thinner films [5,6]. Radiation interacts with polymers in two ways: chain scission, which results in reduced tensile strength and elongation; and crosslinking, which increases tensile strength but reduces elongation. Both reactions occur simultaneously, but one is usually predominant, depending upon the specific polymer and additives involved. A linear correlation of polymer conductivity with the applied dose was reported [3,1]. Two non-conducting forms of polyaniline were used as a detecting device for low-dose ionization radiation, whereas the conducting form manifested itself as a possible high dose detector [1]. The radiation interaction was mainly due to an oxidation process of the main polymer
116
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
chain yielding a conductivity alteration. Ionising radiation also induced a doping state in the polymer main chain structure similar to that found in the conventional acid doping process [1]. Polyaniline/poly acid acrylic composite thin films were used as ionization radiation sensors. Preliminary gamma radiation interaction with composite showed a linear response that can be used in the development of real-time radiation sensors for dose range from 0 to 5000 Gy [1]. Polyaniline optical dosimeter for 60 Co gamma radiation was based on colour change of 40 nm thick nanofilms [7]. The deep blue colour, which is a characteristic of undoped polyaniline film before irradiation, became subsequently green as the film was irradiated to a dose range from 0 to 10 kGy. Polyvinylidene fluoride (PVDF) is a long chain highmolecular-weight polymer with predominant repeating unit established as (–CH2 –CF2 –). This semi-crystalline material with a melting point of 338 ◦ C is widely used for various technical/engineering applications. Several types of sensors based on PVDF film were designed and tested, including laser radiation sensors, a light pressure transducer, an ion radiation sensor, and a multifunctional alarm [8]. Pure and doped PVDF films were employed in a pyroelectric detector measuring short laser pulses in the sub-nanosecond range [9]. Semi-crystalline PVDF, irradiated with different ions in the range 100 keV–100 MeV, exhibited a strong change in the crystalline structure, as detected by X-ray diffraction measurements and calorimetric analysis [10]. The relationship between the crystallinity of PVDF and electron radiation effects was studied with differential scanning calorimetry and X-ray diffraction [11]. It was reported that at doses lower than 400 kGy, the crystallinity of PVDF increased slightly with the increase in absorbed dose, and then decreased slowly. A decrease in the crystallinity was accelerated by the increase in the absorbed dose at higher dose zone [11]. At ion doses higher than 300 kGy, PVDF showed a degradation in the crystalline structure, reduction of hydrogen and fluorine concentration and the formation of carbon reticulation [10]. This work focuses on the development of low-dose gamma radiation sensor based on screen-printed PVDF thick films. To enhance the conductivity of the overall mixture, carbon was added into PVDF, at the amount close to percolation threshold. Gamma radiation-induced changes in the electrical properties of PVDF capacitors were the bases for dose estimation. Conducting composites based on conductive particles in a host matrix are widely used owing to their unique electrical and mechanical properties [12]. Most polymer thick films are typical insulators and the conductivity of the composite materials is predominantly dependant on the content and properties of the filler as well as on composite structure. New properties may originate from specific interaction between the material and the added amount of carbon in addition to the particular arrangement of the filler particles in the host matrix [13]. 2. Experimental procedure The main ingredient of the paste was PVDF powder, supplied by Sigma Aldrich. To improve the conductivity of thick films,
Fig. 1. Schematic structure of capacitor structure, where 1 is alumina substrate; 2 is Ag electrodes and 3 is carbon-filled PVDF thick film.
6 wt.% of carbon black powder was also added into the paste. It was shown that pure PVDF is tolerant to radiation of up to ∼300 kGy. PVDF used in medicine has radiation tolerance level of around 1000 kGy. In our experiments we used Carbon doping to make devices sensitive to much lower doses of gamma radiation. In practise, pure PVDF devices with the same configuration were too insulative, with no current passing through the circuit under the applied voltage of up to 10 V. Higher voltages are not feasible. Contrary, pure carbon paste is too conductive. Carbon black was supplied by Sigma Aldrich and had a particle size of 42 nm. This was added to 7 wt.% ethyl cellulose and 1 wt.% of surfactant (supplied by Lennox Ltd) to surfactant to achieve a homogeneous dispersion of carbon particles. The surfactant was a polymeric amphiphile composed of anchoring groups, which are adsorbed onto the surface of the carbon black particles. The chemical structures of the hypermer dispersing agents and also their large molecular weights provide a steric barrier to minimise particle–particle interaction and thus reduce re-agglomeration of carbon black [14]. Finally, Terpinol-␣ (Sigma Aldrich) was used as the solvent. The samples were fabricated by screen-printing on alumina substrates. Active area of capacitive structures measured 2 mm × 3 mm, while Ag paste was chosen for electrodes material (see Fig. 1). DEK RS 1202 automatic screen printer was used for device fabrication. After carbon-filled PVDF paste was printed onto bottom electrode, it was dried in air and cured at 100 ◦ C for 30 min and then at a peak temperature of 170 ◦ C for 10 min. In-house printing of resistors using thick-film technology can result in resistor tolerances of ±20%. At industrial manufacturing conditions considerably higher sample reproducibility can be achieved. When using screen-printing for thick film manufacture, there is a limit of how thin the layer can be. It depends on the material itself, its particle size, on the mesh, through which the paste is squeezed during printing, on the number of screen-prints, and on the firing conditions (normally layers are thinner after drying). At least few layers of carbon-filled PVDF paste were required to ensure repeatability of the devices and, more importantly, to avoid possible pinhole formation, which could lead to the shortage of the device when electrodes are applied. A number of samples with two different thicknesses were manufactured. A Sloan Dektak 900051 Profilometer was used to determine the average thickness of the dielectric layers, which was found to be 23.97 and 39.12 m for samples #1 and #2,
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
117
capacitive interface circuitry. The system consists of on-chip temperature sensor, 24-bit SD modulator, digital filter, voltage regulator and serial interface—all integrated in one module. The module can operate with a single power supply of 2.7 V. The output information from the digital filter is read by the microcontroller unit through a serial interface part. Capacitive sensors exhibit a change in capacitance in response to a change in physical stimulus. Most of the designed capacitive systems are based on converting the capacitance to voltage first. Then this voltage is converted into digital domain with high precision analog-todigital converter (ADC). Detailed information on the wireless capacitive module can be found elsewhere [15]. 3. Results and discussion
Fig. 2. The picture of C-PVDF thick film surface.
respectively. A surface roughness of both films was about 1 m, therefore thick film Ag electrodes were used. Fig. 2 shows the picture of C-PVDF thick film surface, which was taken using Olympus Microscope BX60. To investigate the effect of radiation on the crystalline structure of the material, qualitative X-ray powder diffraction (XRD) was performed using a Philips X’pert PRO multi purpose diffractometer (MPD) X-ray diffractometer PW3050/60 θ–θ (Philips, Eindhoven, Netherlands) within a scan range of 5–70◦ (2θ). The CuK␣ diffractometer anode was run under a tension of 40 kV and a current of 35 mA. An X’Celerator strip detector was used to collect the diffracted data. A glancing angle of 3◦ (2θ) was applied in order to prevent anomalous data being obtained from the glass slide. 137 Cs (0.662 MeV) disk-type source with an activity of 370 kBq (provided by AEA Technology QSA GmbH as a standard reference gamma radiation source) was used to expose the samples to ␥-radiation. The radioactive gamma-emitting element (3.18 mm × 5 mm) was encapsulated into a 2 mm thick high strength epoxy resin (diameter 25 mm) to shield any accompanying -radiation. The source was held at a distance of 1 cm from the device under investigation at an angle of incidence of 0◦ . A set of irradiations were performed, changing the exposure time and hence the dose. Values of radiation damage were estimated from changes in the current–voltage characteristics of the C-PVDF capacitors. The I–V measurements were taken after each irradiation. To investigate the effect of carbon loading on the radiation sensitivity of the PVDF thick films, device having identical structure to samples #1 and #2, but 4 wt.% of Carbon, was tested. This is denoted as sample #3. Reduced carbon loading resulted a decreased conductivity of thick film sensor. Therefore, capacitance versus dose measurements were used to trace the effect of gamma radiation on the sample #3 electrical properties. The changes in the values of capacitance with radiation were measured wirelessly in real-time via specially designed
PVDF is a semicrystalline ferroelectric polymer, which can crystallise into four different crystal forms: ␣, , ␥, and ␦ depending upon the crystallisation conditions. The most common form is the ␣-phase, which is non-polar. The ␣-phase can be transformed to highly polar -phase by mechanical deformation and pressure [16]. Fig. 3 depicts the XRD scan for PVDF thick film on glass substrate. This spectrum indicates the presence of semi-crystalline structure. As expected, the film is predominantly ␣-phase. The main peaks, with their ‘h k l’ values in brackets, associated with ␣-phase are at 2θ values of 17.9◦ [1 0 0], 18.4◦ [0 2 0], 20.1◦ [1 1 0] and 26.7◦ [0 2 1] [17,18]. No phase transition was recorded due to the effect of radiation dose of ∼10 mGy, e.g. the XRD scan remained identical to Fig. 3. The electrical properties of mixtures containing carbon black are widely explained and experimentally supported within the framework of inter-particle tunnelling conduction and/or the framework of classical percolation theory [19–21]. Percolation theory considers, in the simplest case, the formation of clusters in the volume of a matrix. Those clusters, after reaching an appropriate concentration of filler particles, could form an infinite network. In the case of conducting filler particles, the conductivity in the system should increase dramatically after reaching the condition, known as the percolation threshold [20]. Basic models, which consider the dispersion of spherical particles throughout an insulating matrix, show that the conductivity (σ) of the system varies so that σ = σ 0 (ν − νc )t , where ν is the volume fraction of conductive filler and νc is the critical volume fraction, which occurs at the percolation threshold. This important point coincides with the formation of the first continuous,
Fig. 3. XRD patterns of carbon-filled PVDF thick film on a glass substrate.
118
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
Fig. 4. Changes in current–voltage plots with dose for C-PVDF sample #1. Fig. 5. Changes in current–voltage plots with dose for C-PVDF sample #2.
conducting pathway, which spans the entire system. For volume fractions above the percolation threshold, the conductivity of the composite is closer to that of the filler, which indications that an unbroken conduction networks exist [12]. The exponent t is assumed to be universal and conveys the changing conductivity with conducting filler concentration, while σ 0 is the conductivity of the fillers or conductive phase. From the above description, it may be understood that the percolation threshold corresponds to an increase in conductivity of many orders of magnitude and so the sensitivity of the entire system is highly dependant on the concentration of conducting particles present [12]. The electrical conductivity of carbon black containing composites was found to depend on the structure of the carbon black particles and to deviate from the expectations of classical percolation theory [19]. In this study, reproducible samples with a given fraction of carbon black, prepared under the same processing conditions have been achieved. Hence, a very similar spatial distribution of carbon black should be expected. Generally, a trend can be noticed that the larger the inhomogeneity in carbon black distribution, the smaller is the effective conductivity of the composite. This is why mixing processes require proper monitoring, e.g. automated shear mixing is recommended. Fig. 4 displays changes in current–voltage plots with dose for PVDF sample #1, i.e. having a thickness of 23.97 m. One may see that a monotonic tenfold increase in the values of current was caused by irradiation with gamma dose of 228 Gy. A higher dose of 342 Gy was critical to the performance of these devices and a considerable decrease in the values of current was recorded. Therefore a range of 0–228 Gy can be considered as the working dose range for these carbon-filled PVDF capacitors. Annealing of #1 thick films for 4 h in a conventional oven at 80 ◦ C was found to fully restore their original properties after the irradiation. This suggests that these films can be reused. However, if the films are subjected to much higher doses of radiation, some permanent defects can occur. To remove them, elevated temperatures are needed, which in turn can change the structure and properties of the polymer thick film. Fig. 5 displays changes in current–voltage plots with dose for a thicker sample #2. These films showed an increase in the values of current with increasing gamma dose up to a level of 798 Gy. This level is higher than 228 Gy of the sample #1.
Unlike sample #1, there was no rapid decrease in the values of current recorded for the sample #2, when it was irradiated with a dose higher than 798 Gy. Instead, a slow monotonic decrease was monitored. To trace and compare the performance of these two samples with different thicknesses, see Fig. 6, which shows the dependence of normalised currents (I − I0 )/I0 with radiation dose for both samples at the same fixed applied voltage of 3 V. It is important to mention that at higher applied voltages, the effect of gamma radiation on the electrical properties of carbonfilled PVDF thick film capacitors was more pronounced, e.g. the changes in the values of current were bigger. However, highapplied voltage causes additional Joule heating, leading to an increased migration of conducting particles through the polymer matrix. Moreover, Joule heating caused metal inclusions from the Ag electrode material into the polymer sensing layer. As a result, an electroforming effect was recorded at 12 V for asprinted PVDF thick film #1, whereas films irradiated with a dose of 171 Gy showed strong electroforming effect already at a lower voltage of 5 V. Accordingly, after electroforming takes place, the device is no longer considered reliable for dosimetry purposes. Fig. 7 illustrates radiation-induced changes in the value of capacitance of C-PVDF sample #3, e.g. with 4 wt.% carbon doping. Irradiation of this sensor with doses from 1.15 to 2.5 mGy
Fig. 6. Dependence of normalised currents (I − I0 )/I0 with radiation dose at a fixed voltage of 3 V for samples #1 and #2.
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120
Fig. 7. Changes in the value of capacitance of C-PVDF sample #3 with radiation.
caused a considerable increase in the values of its capacitance from 2.92 to 12.37 pF. The value of 1.15 mGy is regarded here as minimum detectable dose (MDD), since no measurable changes with radiation were recorded prior to that [22]. However, MDD is affected by material composition and device structure, including thickness. After a dose of 2.5 mGy C-PVDF sensor with 4 wt.% carbon showed saturating response, therefore a dose range of 1.15–2.5 mGy is considered as a working dose range for this particular radiation sensor. Generally, the ␥-radiation-induced changes in the electrical characteristics of the samples were found to be similar to the dose response of most materials used in thermoluminescence (TL) dosimetry [23]. They usually show a linear, then supralinear, and finally a saturating response, beyond which damage occurs with the increase in dose. The details of the experimentally measured dose response depend on a large number of parameters, related to the instruments, radiation field and fabrication process [23]. Differences in material properties could determine the nature of changes in the supralinear region on the electrical parameters, under radiation exposure. However, it is the linear response region, which is preferable in radiation dosimetry. All these effects should be taken into consideration, when designing a radiation sensing system, as one has to compromise between a higher response resolution and wider working dose range. 4. Conclusion The aim of this work is to develop cost-effective sensors for personnel dosimetry application. Each device has its limitation, e.g. max applied voltage, max/min operating temperature, load and so forth. For the devices under investigation, one of the most important parameters is the working dose range, beyond which the device is considered as damaged. Device response is correlated with the real applied dose only in the working dose range of that particular device. Unfortunately, no correlation is found after these devices have been damaged. The electrical properties of PVDF based screen-printed thick films were explored in terms of gamma radiation influence. A polymer paste was filled with 4 and 6 wt.% of carbon black
119
to increase the conductivity of the device. Moreover, increased conductivity resulted in enhanced sensitivity of the PVDF thick films to ␥-rays. A monotonic tenfold increase in the values of current was recorded for samples #1 having thickness of 23.97 m and 6 wt.% of carbon, after the irradiation with a dose of 228 Gy. Annealing of these samples for 4 h in a conventional oven restored their properties after they were irradiated. This indicates the possibility to reuse the films. Thicker PVDF films (sample #2) could sustain higher radiation dose of 798 Gy and slow decrease in the values of current was monitored with further increase in dose. Gamma radiation sensitivity of counterpart PVDF thick films with 4 wt.% carbon doping was studied via capacitance-dose measurements in real time. Irradiation of this sensor with doses from 1.15 to 2.5 mGy caused a considerable monotonic increase in the values of its capacitance from 2.92 to 12.37 pF, whereas the value of 1.15 mGy is regarded as minimum detectable dose. Combining sensors with various levels if carbon doping into sensor array would enhance radiation sensitivity of the overall system. Based on the experimental results, the manufacture of polymer based thick film structures should be considered as a cost-effective alternative to traditional wafer-based fabrication techniques, as it can produce sensors with real time dose measurement of ␥-radiation at room temperature. Acknowledgements Funding was received from the Irish Research Council for Science, Engineering and Technology: funded by the National Development Plan. References [1] A.P. Lima Pacheco, E.S. Araujo, W.M. de Azevedo, Mater. Character. 245 (2003) 50. [2] K. Arshak, O. Korostynska, Mater. Sci. Eng. B 224 (2004) 107. [3] K. Arshak, A. Arshak, S. Zleetni, O. Korostynska, Nucl. Sci., IEEE Trans. 2250 (2004) 51. [4] K. Arshak, O. Korostynska, F. Fahim, Sensors 176 (2003) 3. [5] K. Arshak, O. Korostynska, IEEE Sens. 717 (2003) 3. [6] E. Atanassova, A. Paskaleva, R. Konakova, D. Spassov, V.F. Mitin, Microelectron. J. 553 (2001) 32. [7] J.M.G. Laranjeira, H.J. Khoury, W.M. de Azevedo, E.A. de Vasconcelos, J. da Silva, Phys. E: Low-Dimensional Syst. Nanostruct. 666 (2003) 17. [8] W. Shuduo, Polyvinylidene fluoride film sensors and applications, in: Proceedings, Berlin, Germany, 1992, pp. 923–928. [9] B. Elling, W. Kuenstler, R. Danz, D. Geiss, W. Bohmeyer, Detection of laser radiation using PVDF-films, in: Proceedings of the 6th International Symposium Electrets, 1988, pp. 399–403. [10] L. Torrisi, G. Ciavola, G. Foti, R. Percolla, Nucl. Instrum. Methods Phys. Res., Sect. A: Accel., Spectrom., Detectors Assoc. Equip. 361 (1996) 382. [11] S. Hongbing, X. Liqing, C. Jun, W. Haiyan, Sci. China, Series A: Math., Phys., Astron. 438 (1998) 41. [12] J. Vilc´akov´a, P. S´aha, O. Quadrat, Eur. Polym. J. 2343 (2002) 38. [13] J. Barkauskas, A. Vinslovaite, Mater. Res. Bull. 1437 (2003) 38. [14] K. Arshak, E. Moore, L. Cavanagh, J. Harris, B. McConigly, C. Cunniffe, G. Lyons, S. Clifford, Compos. Part A: Appl. Sci. Manuf. 487 (2005) 36. [15] K. Arshak, O. Korostynska, E. Jafer, A. Arshak, D. Morris, E. Gill, Gamma Radiation Sensing Using ZnO and SnO2 Thick Film Interdigitated Capac-
120
[16]
[17] [18] [19]
O. Korostynska et al. / Materials Science and Engineering B 141 (2007) 115–120 itors, in: Proceeding of the IEEE ISSE, Dresden, Germany, 2006, pp. 495–500. K. Arshak, C. Ryan, M. Campion, Pressure induced piezoelectric/spl beta/phase crystallization in screen printed polymer thick films, in Proceedings, 2002, pp. 411–414. A. Tawansi, A.H. Oraby, S.I. Badr, I.S. Elashmawi, Polym. Int. 370 (2004) 53. L. Priya, J.P. Jog, J. Appl. Polym. Sci. 2036 (2003) 89. I. Balberg, Carbon 139 (2002) 40.
[20] G. Grimmett, Percolation, second ed., Springer Verlag, Berlin, 1999. [21] A. Dziedzic, A.A. Snarskii, B.W. Licznerski, Some remarks about temperature dependence of resistance in percolation system, in: Proceedings of the SPIE Poland, vol. 2780, 1995, pp. 95–99. [22] K. Arshak, O. Korostynska, Advanced Materials and Techniques for Radiation Dosimetry, Artech House, Boston, 2006. [23] Y.S. Horowitz, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 68 (2001) 184.