Radiation Effects and Defects in Solids Thermal

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Jan 31, 2011 - Also, the proton irradiation in the fluence range 7.5 × 1013–5 × 1015 ... the refractive index, transmission of the samples and any color ... Downloaded By: [Nouh,] At: 05:01 17 February 2011 ... formation of C−C bonds, and auto-oxidation occurs in the presence of oxygen. ..... fluence is shown in Figure 2(b).
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Radiation Effects and Defects in Solids

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Thermal, electrical and optical properties of proton-irradiated Makrofol DE 7-2 nuclear track detector S. A. Nouha; M. H. Abdel-Salamb; Yasmine E. Radwanb; S. S. Fouadb a Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt b Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt First published on: 31 January 2011

To cite this Article Nouh, S. A. , Abdel-Salam, M. H. , Radwan, Yasmine E. and Fouad, S. S.(2011) 'Thermal, electrical and

optical properties of proton-irradiated Makrofol DE 7-2 nuclear track detector', Radiation Effects and Defects in Solids, 166: 3, 178 — 189, First published on: 31 January 2011 (iFirst) To link to this Article: DOI: 10.1080/10420150.2010.550006 URL: http://dx.doi.org/10.1080/10420150.2010.550006

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Radiation Effects & Defects in Solids Vol. 166, No. 3, March 2011, 178–189

Thermal, electrical and optical properties of proton-irradiated Makrofol DE 7-2 nuclear track detector S.A. Nouha *, M.H. Abdel-Salamb , Yasmine E. Radwanb and S.S. Fouadb a Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt; b Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt

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(Received 9 August 2010; final version received 17 December 2010 ) Samples from sheets of the polymeric material Makrofol DE 7-2 have been exposed to 1 MeV protons of fluences in the range 2.5 × 1013 –5 × 1015 p/cm2 . The resultant effect of proton irradiation on the thermal properties of Makrofol has been investigated using thermogravimetric analysis and differential thermal analysis (DTA). The onset temperature of decomposition To and the activation energy of thermal decomposition Ea were calculated, and the results indicated that the Makrofol detector decomposes in one weight loss stage. Also, the proton irradiation in the fluence range 7.5 × 1013 –5 × 1015 p/cm2 led to a more compact structure of Makrofol polymer, which resulted in an improvement in its thermal stability with an increase in the activation energy of thermal decomposition. The variation of transition temperatures with proton fluence has been determined using DTA. The Makrofol thermograms were characterized by the appearance of an endothermic peak due to the melting of the crystalline phase. The melting temperature of the polymer, Tm , was investigated to probe the crystalline domains of the polymer. At a fluence range of 7.5 × 1013 –5 × 1015 p/cm2 , the defect generated destroys the crystalline structure, thus reducing the melting temperature. In addition, the V –I characteristics of the polymer samples were investigated. The electrical conductivity was decreased with the increasing proton fluence up to 5 × 1015 p/cm2 . Further, the refractive index, transmission of the samples and any color changes were studied. The color intensity E was greatly increased with the increasing proton fluence and was accompanied by a significant increase in the red and yellow color components. Keywords: proton irradiation; Makrofol; thermal; electrical; optical properties

1.

Introduction

Polymers are a familiar part of everyday life that have found widespread application in many domains of techniques, especially in micro-electronics fabrication and space and nuclear technologies. It is obvious that the primary physical interaction of radiation with solid-state nuclear track detectors produces specific damage known as chemical bond scission, free radicals and consecutive cross-linking. These are accompanied by changes in the chemical, structural, geometrical, optical and electrical properties of the polymer (1–3). The obtained chemical system is characterized by new functional groups, a different backbone with different electronic structures and different physical properties (4). There are several applications of ion-irradiated polymers, such as microelectronics and biosensors production technologies. Dramatic changes in the *Corresponding author. Email: [email protected]

ISSN 1042-0150 print/ISSN 1029-4953 online © 2011 Taylor & Francis DOI: 10.1080/10420150.2010.550006 http://www.informaworld.com

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radiation-induced damage processes may occur if swift heavy ion irradiation is performed, instead of classical-condition irradiation such as by electron beams or gamma rays. The mechanism of the interaction between protons and polymers is rather complicated. Many processes, such as the production of primary and secondary radicals, lead to the formation of double bonds, the transformation of C−C bonds, and auto-oxidation occurs in the presence of oxygen. These reactions depend on the proton dose (5). For polycarbonates irradiated with a 200 keV proton beam, the refractive index was found to be an increasing function of the dose (6). Proton irradiation can also modify the track registration properties of polycarbonates (7). The effects of proton irradiation on different polymers have already been reported (8–13). In our previous work (14), we studied the structural and mechanical modifications induced by proton irradiation in a CR 6-2 polymer. The results revealed that the proton irradiation causes intermolecular cross-linking and allows the formation of covalent bonds between different chains, leading to an improvement in the thermal, optical and mechanical properties of the CR 6-2 polymer. Thus, as an extension to our previous work on the modifications in thermal and optical properties of polycarbonates by 1 MeV protons (15), the present work deals with the modifications induced in thermal, electrical and optical properties of Makrofol DE 7-2 polycarbonate irradiated in the fluence range 2.5 × 1013 to 5 × 1015 p/cm2 .

2.

Experimental

2.1. Samples Makrofol DE 7-2 is a translucent polycarbonate film, supplied with a velvet first surface and a very fine matte second surface. Its chemical composition is C16 H14 O3 , it is manufactured by Farbenfabriken Bayer A.G., Leverkusen (Germany), and has an average thickness of 300 μm and a density 1.2 g/cm3 . 2.2.

Irradiation facilities

The Makrofol samples were exposed to a 1 MeV proton beam at the Ion Beam Center, University of Surrey, UK. The current density of the proton beam was 0.05 μA/cm2 and the beam diameter was 1.8 mm. The irradiation was carried out at fluences in the range 2.5 × 1013 to 5 × 1015 p/cm2 , with the sample held at a vacuum of 10−6 torr. 2.3. Analysis of the irradiated samples 2.3.1. Thermal properties Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on irradiated and non-irradiated Makrofol samples using the TGA and DTA apparatus model Shimadzu-50 with platinum cells. α-Al2 O3 was used as a reference material for DTA measurements. Thermal experiments were carried out on all samples at a heating rate of 10◦ C/min, with nitrogen as a carrier gas at a flow rate of 30 mL/min. 2.3.2. Electrical properties The samples used in electrical conductivity measurements have the form of squares of length, width and thickness 1, 1 and 3 × 10−2 cm, respectively. The electrodes were made of silver paste,

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covering the two major surfaces of the samples. A specially-designed cell in which the sample is fixed between two parallel plates of brass, isolated from each other by Teflon, was used. The current was measured using a digital electrometer (Keithley 616). This electrometer is essentially used for measuring weak currents in high resistivity samples. The DC electrical conductivity was determined from the relation DC conductivity = I d/V A, where d is the thickness of the examined sample in cm, A the cross-sectional area of the sample in cm2 , V the applied voltage (in V) and I is the current (in A).

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2.3.3. Optical properties 2.3.3.1. Refractive index. The refractive index measurements were carried out using an Abbe refractometer (Reichert; mark II, Model-10480, New York, USA). The accuracy of measuring the values of refractive indices, surface temperature of the prism and the wavelength of the light used were ±0.0001, 20–22◦ C and 5893 Å, respectively. Several values were measured for the same sample and the average value was considered. 2.3.3.2. Color difference measurements. The transmission measurements were carried out using a Shimadzu UV–vis–NIR scanning spectrophotometer, type 3101 PC. This unit measures in the wavelength range from 200 to 3000 nm. The Commission International de E’Claire (CIE units x, y and z) methodology was used in this work for the description of colored samples. 2.4.

Determination and calculation of the tristimulus values

Vision scientists have created a special set of mathematical lights, X, Y and Z, to replace actual red, green and blue lights. The color matching functions for the X, Y and Z lights are all positive numbers and are labeled x, ¯ y¯ and z¯ . Every color can be matched using the appropriate amount of X, Y and Z light. The amount of X, Y and Z light needed to match a color are called its tristimulus values. The CIE tristimulus values for a transmitting sample are calculated by adding the product of the spectral power distribution of illuminant, the transmittance factor of the sample and the color matching functions of the observer at each wavelength of the visible spectrum, as shown in the following equations:  X=k P (λ)x(λ)T ¯ (λ),  Y =k P (λ)y(λ)T ¯ (λ),  Z=k P (λ)¯z(λ)T (λ), k=

100 , P (λ)y(λ) ¯

where P (λ) is the value of the spectral power distribution of the illuminant at the wavelength λ. T (λ) is the transmittance factor of the sample at the wavelength λ, and x(λ), ¯ y(λ) ¯ and z¯ (λ) are the CIE color matching functions for the standard observer at the wavelength λ. The factor k normalizes the tristimulus value so that Y will have a value of 100 for a perfect white diffuser. 2.5.

The 1976 CIE L∗ a∗ b∗ (CIELAB) color space

A weakness of the CIE X, Y and Z color space is its lack of visual uniformity. Creating a uniform color space would have two major advantages. It would allow plots showing the perceptually

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relative positions of two or more colors in a color space, and it would facilitate the creation of a good color difference ruler between two samples. The 1976 CIE L∗ a ∗ b∗ (CIELAB) color space is widely used in the paint, plastic and textile industries. L∗ correlates with the perceived lightness in CIELAB color space. A perfect white would have an L∗ of 100, and a perfect black would have an L∗ of 0. The coordinates a ∗ and b∗ have their history in the opponent color theory. It was proposed that three pairs of opposing color sensations produce all colors: red and green; yellow and blue; and black and white. The CIELAB coordinate a ∗ correlates with red (+a ∗ ) and green (−a ∗ ), while the coordinate b∗ correlates with yellow (+b∗ ) and blue (−b∗ ). The CIELAB L∗ , a ∗ and b∗ coordinates are calculated from the tristimulus values according to the following equations   Y/ − 16, L∗ = 116f Yn      Y X ∗ a = 500 f −f , Xn Yn      Z Y b∗ = 200 f −f , Yn Zn where X, Y and Z are the tristimulus values and the subscript n refers to the tristimulus values of the perfect diffuser for the given illuminant and standard observer; f (X/Xn ) = (X/Xn )1l3 for values of (X/Xn ) greater than 0.008856 and f (X/Xn ) = 7.787(X/Xn ) + 16/116 for values of (X/Xn ) equal to or less than 0.008856; and the same with Y and Z, replacing X in turn. The CIELAB color difference, E, is given by (16, 17): E = [(L∗1 − −L∗2 ) + (a1∗ − a2∗ ) + (b1∗ − b2∗ )]1/2 . The subscripts 1 and 2 refer to the irradiated and non-irradiated samples.

3.

Results and discussion

3.1. Thermal analysis techniques 3.1.1. Thermogravimetric analysis TGA was performed on Makrofol samples in the temperature range from room temperature up to 600◦ C, at a heating rate of 10◦ C/min. Figure 1(a) shows the TGA thermograms for the nonirradiated and irradiated samples. It is clear from the figure that the Makrofol detector decomposes in one main breakdown stage. Using the TGA thermograms, the values of the onset temperature of decomposition To , the temperature at which the decomposition starts, were calculated. Figure 1(b) shows the variation of To with the proton fluence. The figure shows that To decreases until a minimum value around the 7.5 × 1013 p/cm2 irradiated sample due to degradation (i.e. preferentially chain scission), followed by an increase with an increase in the proton fluence up to 5 × 1015 p/cm2 , due to the cross-linking process. 3.1.2. Activation energy of thermal decomposition (Ea ) Evaluation of the activation energy of thermal decomposition is useful for studying the thermal stability of materials. Various thermogravimetric methods based on either the rate of conversion or the heating rates have been reported to determine the thermal kinetic parameters. The method

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Weight (mg)

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7 6 5 4 3 2 1 1. 7.5E13 5. 7.5E14 6. 5E13 2. 2.5E14 7. 2.5E13 3. 5E14 4. Non-irradiated

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Figure 1. (a) TGA thermograms of the non-irradiated and irradiated Makrofol samples, measured in the temperature range from room temperature up to 600 ◦ C, at a heating rate of 10 ◦ C/min. (b) Variation of the onset temperature of decomposition To with the proton fluence. (c) Variation of the activation energy of thermal decomposition Ea with the proton fluence.

proposed by Horowitz and Metzger (18) has been used in the present study for the measurement of the activation energy of thermal decomposition. Using the TGA curves shown in Figure 1(a), values of Ea were calculated for the non-irradiated and irradiated Makrofol samples and are shown in Figure 1(c). The figure shows that Ea decreases to a minimum value around the 7.5 × 1013 p/cm2 irradiated sample, then increases with fluence up to 5 × 1015 p/cm2 . The interpretation of these results may be that, at the dose range 2.5–7.5 × 1013 p/cm2 , initial scission occurs. This is reflected in a decrease in Ea of the polymer samples. In the dose range 7.5 × 1013 to 5 × 1015 p/cm2 , the free radicals formed due to scission are chemically active and can be used in some chemical reactions that lead to the cross-linking mechanism (14). 3.1.3. Differential thermal analysis DTA was performed in the temperature range from room temperature up to 400 ◦ C at a heating rate of 10 ◦ C/min; the obtained thermograms are shown in Figure 2(a). The Makrofol samples are characterized by the appearance of an endothermic peak at the melting temperature Tm . The values of these melting temperatures could be calculated, and the variation of Tm with the proton fluence is shown in Figure 2(b). The figure shows that Tm increases up to a maximum value around 7.5 × 1013 p/cm2 and then decreases with an increase in the proton fluence up to 5 × 1015 p/cm2 .

Radiation Effects & Defects in Solids

(a) 21

1. Non-irradiated 2. 1 E15

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Figure 2. (a) DTA thermograms of the non-irradiated and irradiated Makrofol samples, measured in the temperature range from room temperature up to 400 ◦ C, at a heating rate of 10 ◦ C/min. (b) Variation of the melting temperature Tm with the proton fluence.

Usually, the increase in the melting temperature of a polymer is attributed to cross-linking and vice versa. The apparent discrepancy between the dependence of To and Tm on dose results from the fact that Tm is sensing the crystalline domains of the polymer. It is possible to speculate that at low doses, the thickness of the crystalline structures (lamellae) is increased. At higher doses, defect generation splits the crystals, depressing the melting temperature. For such doses, the decrease in the polymer length also contributes to the shift of Tm towards lower temperatures. Similar results were obtained by Nasef et al. (19) when they studied the electron beam irradiation effects on partially-fluorinated polymer films, where they found that cross-linking inhibits the crystallization and this is coupled with a gradual decease in Tm with the irradiation dose. The same was also found by Nouh and Hegazy (20) when they studied the effect of neutron irradiation on the thermal properties of cellulose nitrate. 3.2. Electrical properties Figure 3 shows the V –I characteristics for the non-irradiated and irradiated Makrofol samples, at a constant temperature of 296 K, in the voltage range 0–500 V. It is observed that the current increases on increasing the voltage for each sample. The values of electric conductivity were calculated and plotted in Figure 4 as a function of the proton fluence. It is observed that proton irradiation up to 5 × 1015 p/cm2 leads to a decrease in conductivity of the Makrofol samples. This can be attributed to the fact that cross-linking reduces crystallinity and induces further lattice defects that may act as scattering centers and energy barriers for the flow of electric current, thus reducing the conductivity (21). 3.3. Optical properties 3.3.1. Refractive index The refractive indices of solid sheets of Makrofol were measured. Figure 5 shows the variation of the refractive index with the proton fluence. The refractive index showed a decrease in magnitude

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V –I characteristics of the non-irradiated and irradiated Makrofol samples.

Conductivity (ohm–1 cm–1)

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Fluence (p/cm2) Figure 4. Variation of the DC conductivity with the proton fluence.

to a minimum value at 7.5 × 1013 p/cm2 , followed by an increase on increasing the fluence up to 5 × 1015 p/cm2 . This behavior can be explained in terms of degradation and cross-linking induced by proton irradiation. Such behavior facilitates the formation of free radicals that are chemically active. This allows the formation of covalent bonds between different chains (cross-linking), and

Radiation Effects & Defects in Solids

Refractive index

1.592 1.590 1.588 1.586 1.584 1.582 1.580 1.578 1.576 1E+13

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in turn minimizes the anisotropic character of the Makrofol polymer, leading to the increase in the refractive index. These results are in good agreement with those obtained by Shams-Eldin et al. (22), who illustrated that the incident radiation activates the main polymer chain, implying a main chain scission which results in a decrease in the refractive index. The same effect was also investigated by Ranby and Rebek (23).

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Figure 5. Variation of the refractive index with the proton fluence.

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Figure 6. The transmittance spectra of non-irradiated and irradiated Makrofol samples, measured in the wavelength of 300–2300 nm.

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Color changes

The transmission spectra of the irradiated and non-irradiated Makrofol samples, in the wavelength range 300–2300 nm, have been investigated. The spectra appeared, for all Makrofol samples, as a band with different intensities (as shown in Figure 6). Using the transmission data, both the tristimulus values and chromaticity coordinates were calculated. Figure 7 shows the variation of tristimulus values (X, Y , Z) with the proton fluence. From the figure, it is clear that X, Y and Z exhibited the same trend, where they decreased on increasing the proton fluence up to 5 × 1015 p/cm2 . Figure 8 shows the variation of chromaticity coordinates (x, y, z) with the proton fluence. From the figure, it is clear that x and y exhibited the same trend, where they increased with the increasing the proton fluence up to 5 × 1015 p/cm2 . The chromaticity coordinate z exhibited an opposite trend with the proton fluence.

Z

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Figure 7. Variation of the tristimulus values (X, Y and Z) with the proton fluence.

Chromaticity coordinates

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Figure 8. Variation of the chromaticity coordinates (x, y and z) with the proton fluence.

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12 b*

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Figure 9. Variation of the color intercepts (L∗ , a ∗ and b∗ ) with the proton fluence.

The variation of the color intercepts (L∗ , a ∗ and b∗ ) with the proton fluence is shown in Figure 9. The accuracy in measuring L∗ is ±0.05, and is ±0.01 for a ∗ and b∗ . It can be seen that the color parameters a ∗ , b∗ and L∗ were significantly changed after exposure to proton irradiation. The blue (−b∗ ) color component of the non-irradiated film was changed to yellow (+b∗ ) after exposure to protons up to 5 × 1015 p/cm2 . This is accompanied by a net increase in the darkness of the samples (−L∗ ). At the same time, the red (+a ∗ ) color of the non-irradiated sample was also increased by the proton fluence. The color intensity E (color difference between the non-irradiated sample and those irradiated with different proton fluences) was calculated and is plotted in Figure 10 as a function of proton 32 27 22 DE

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Color intercepts

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7 1.E+13

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Figure 10. Variation of the color intensity E with the proton fluence.

1.E+16

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fluence. From the figure, it is seen that E increases with the increasing fluence up to 5 × 1015 p/cm2 . This indicates that the Makrofol polymer has a response to color change by proton irradiation. These changes in color can be attributed to the trapping of the excited free radicals that are formed by indirect ionization. Also, the trapped free radicals resulting from radiationinduced rupture of polymer molecules have electrons with unpaired spin; such species may also give optical coloration (17).

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4.

Conclusion

Proton irradiation in the fluence range 7.5 × 1013 to 5 × 1015 p/cm2 leads to a more compact structure of Makrofol polymer, which results in an increase in the onset temperature of decomposition and the activation energy of thermal decomposition. This enhances the scope of this polymer in high-temperature applications. Proton irradiation of up to 5 × 1015 p/cm2 causes cross-linking that induces further lattice defects, which act as scattering centers and energy barriers for the flow of the electric current, thus reducing the conductivity. The non-irradiated Makrofol sample is nearly colorless; however, it showed a significant color sensitivity towards proton irradiation. This appeared clearly in the change in the blue color component of the non-irradiated Makrofol film to yellow, accompanied by a net increase in the darkness of the samples with an increase in the red component.

Acknowledgement The authors gratefully acknowledge the assistance of staff at the University of Surrey Ion Beam Centre in carrying out the irradiation of the samples used in this study.

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