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Optical properties of epoxy-glass microballoons composite
January1996
ELSEVIER
1,
Optical Materials 5 (1996) 69-73
Optical properties of epoxy-glass microballoons composite Y. Ramadin a M. A1-Haj Abdallah a M. Ahmad S.K.J. A1-Ani b, S.G.K. A1-Ani b
a,
A. Zihlif a , * ,
a Physics Department, The Universi~ of Jordan, Amman, Jordan b Physics Department, Women Education College, Baghdad-Universi~, Baghdad, Iraq
Received 24 October 1994; accepted 4 July 1995
Abstract The optical properties in the uv-visible region of epoxy glass composite containing 0, 15, 35 and 55% by weight glass microballoons are reported. The obtained optical data are analysed in terms of absorption formula for non-crystalline materials. Assuming that non-direct transitions in the k-space are involved values of optical band gap (Eopt) were determined. From the Urbach edges, the width of the tall of localized states in the band gap (E) were evaluated at room temperature.
1. Introduction The epoxy-glass microballoons composite used in this study is a relatively new advanced material. It is based on addition of some light fillers as glass microballoons into polymer matrix. The obtained lighter structure is very useful in some specific advanced technology. The tensile mechanical properties and thermal behaviour (hot joints) of this composite were reported elsewhere [ 1 ]. The ac-conductivity and the dielectric properties in temperature range = 293-399 K in frequency range 100Hz-10kHz have been recently studied [2,3]. These properties showed frequency and temperature dependence. It is interesting to investigate the electronic and the optical transitions in such materials in terms of theories of non-crystalline solids. Therefore, this paper is concemed with the optical properties of the given composite in the uv-visible region for epoxy glass microballoons composite. * Corresponding author. 0925-3467/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDIO925-3467(95)00033-X
2. Experimental
2.1. Materials preparation
The glass microballoons were supplied by Shell company in U.S.A. The composite was prepared from the epoxy resin (Epon 828) cured by V-40 and hollow glass spheres of diameter ranges from 10 to 18 txm and typical effective density 0.21 g cm -3. Sheets of different glass microballoons concentration (0, 15, 35, and 55%) were obtained by mechanical mixing the epoxy resin, curing agent and the microballoons. The mixture was stirred carefully, and the test samples were allowed to react at 25°C for 24 hour and cured at 80°C in an oven for three hours. The final step was to postcure the composite at 150°C for two hours. Specimens were slowly cooled in the oven to the room temperature. Four disk-shaped specimens of 20 mm diameter were machined from the composite sheets for optical measurements.
70
Y. Ramadin et al. /Optical Materials 5 (1996) 69-73
o.o
8oo
~(2A)
~
g
.o
¢-.0
Wavelength (nm)
Fig. 1. Optical absorbance for composite specimens with different concentration of glass microballoons.
2.2. Optical measurements
a(co)hto= B ( h t o - Eopt)r,
The optical absorbance (A) is taken at a wavelength (A = 200-850 nm) using Pye-Unicam PU 8800 uvvisible spectrophotometer. A suitable cell was used to hold the specimen under investigation. After correcting for reflection at the first surface the absorption coefficient ( a ) at angular frequency (to) of radiation was calculated using absorption equation
I=Io exp( - at).
(1)
Hence a(to) = 2.303 log ~ = 2.303 A , t
1
t
where Io and I are the incident and transmitted intensity respectively. The actual sample thickness (t) was in the range (0.35 to 1.1) mm, and all measurements were performed at room temperature. 3. Results and discussion
3.1. High absorption At high absorption levels, where a(co) > 10 4 c m - 1, the absorption coefficient for non-crystalline materials has the following frequency dependence [ 3,4]
(2)
where B is a constant and r an exponent which can assume values of 1, 2, 3, 1/2 and 3/2 depending on the nature of the electronic transitions responsible for the optical absorption. Fig. 1 represents the spectra of the optical absorbance for 0, 15, 35 and 55% wt% glass microballoons composite samples while Fig. 2 shows the parameter (ahto) ~/r versus hto in accordance with Eq. (2) and for r = 2. The values of Eop t extrapolated from the linear portions in the high regions are listed in Table 1. A good straight line, however, is obtained with r = 2 (Fig. 2); for all samples and reasonable values of Eopt are obtained. This indicates that the transition energy for electrons is non-direct transitions in k-space. In a recent study on MgO-P205, glasses AI-Ani and Higazy [5,6] have reported two mechanisms for electron transitions for electrons namely, direct allowed (i.e. r = 1/2) transitions and non-direct (i.e. r = 2) transitions in k-space. The obtained results show that Eop=tends to decrease with the filler concentration as expected. The values of the constant B are obtained from the slope of the linear part of Fig. 2 and also listed in Table 1. It can be seen that the values of B are lower than those of the pure polymer and of the order of other glasses [6].
Y. Ramadin et al. / Optical Materials 5 (1996) 69-73
71
3.2. Low absorption
{dE) 112 o, (,eVe~' } i 18
In case of lower (in the range 1 c m - ' to 10 -4 c m - l ) absorption the absorption coefficient or(to) (range 1 cm - I to 104 c m - l ) , is described by Urbach formula [7]:
17 l{ 15
a( to) = Otoexp( hto/ AE),
14
(3)
where ao is a constant and AE is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap. Fig. 3 presents the Urbach plot for the samples in Fig. 1. The values of AE were obtained and also listed in Table 1. The exponential dependence of a(to) on hto for these samples indicates
1'3 12 II
5
I0
Ln~
9 8
7 6
5
4
Eg•
2 , , ,V
/J
-//
= 2,03 e V ]E --2.32 e V E
.- 2.53 eVz 3,5
3 2 ]
0 1.5
rE) 2
25
3
eV
Fig. 2. (aE)~/2 versus the incident photon energy for r = 2 (nondirect transition).
=o.]4ev (II),E 2 0 ]4eV
2.5 [
Table 1 Derived values of A E, the constant B and Eopt obtained from different optical mechanisms (Fig. 2) for four epoxy-glass microballoons samples Samples
Eop, (eV) (ahco) ~/2
Constant B (cm -~ e V - ' )
A E (eV)
0 wt pure: I 15 wt%: II 35 wt%: II1 55 wt%: IV
2.53 2,32 2.03 2.41
1193.4 306.3 190.4 818.0
0.34 0.24 0.60 0.55
/
151.5
Z'
2'5
(i,)~E • o s , v
( , v ~ : o ss.v
3'
eV . (E)
Fig. 3. Natural logarithem of ot versus the incident photon energy for r=2.
72
Y. Ramadin et al. / Ootical Material,~ 5 ( 19961 6Q-7~
Fig. 4. The SEM micrograph for specimens with (a) 5 and (b) 55 wt% glass microballoons.
that they obey Urbach's rule. The values of AE were higher than those reported for non-crystalline materials [8,9]. Those tails become higher as the concentration of the glass balloons increases which is consistance with Eopt variation. The decrease in Eop, with glass content can be understood by considering the mobility gap variation in composites as proposed by Davis and Mott [4]. Since Eop, represents generally the energy difference between the localized states in the valence band and extended states in the conduction band or vice versa, then we assume AEc = AEv. Hence, Eopt + AE represents the mobility gap. However the used composite is a heterogenous material where the electrical conduction mainly depends on structural defects, voids
and impurities which probably makes the Eoptnarrower by increasing the width of band tailing.
3.3. Compositemorphology The SEM micrograph for 5 and 55 wt% glass microballoons composite samples (Fig. 4) display the textural and morphological evolution of the composite as a function of filler content. It is clear that at low concentration, the microballoons are distributed randomly in the matrix with no surface contacts between them, while at high concentration some contacts are existing between the adjacent glass microballoons. A small number of glass microballoons are pulled out from
Y. Ramadin et al. /Optical Materials 5 (1996) 69-73
fractured surfaces which appear with dark spheres through the epoxy matrix. Other features of the composite morphology can also be drawn from the micrographs as the hetrogeneous distribution due to mechanical mixing of glass microballoons with the epoxy resin. The thermal curing may displace the microballoons to stick to each other, especially at high filler concentration. Certainly, this composite morphology plays a rather effective role in the optical absorption process.
4. Conclusion The behaviour of the optical absorption in the uvvisible region for epoxy-glass microballoons composite is studied as a function of filler concentration at room temperature. Two formulae were used to analyse the obtained optical data in high and low absorption ranges. The energy gaps were evaluated in both optical absorption cases.
73
Acknowledgements The authors thank Professor R.J. Farris, and L. Feldman of Umass, Amherst, U.S.A. for their contribution in the composite preparation.
References [ 1 ] A. Zihlif, L. Feldman and R. Farris, J. Mat. Sci. 24 ( 1989 ) 3267. [2] M. Shahin, M. AI-Haj Abdallah, A. Zihlif and R. Farris, Dielectric Properties of Epoxy-Glass Microballoons Composite, Polymer Int., submitted for publication; M. Shahin, M. Sc. thesis, Physics Dept. University of Jordan (1994). [ 3 ] J. Tauc, Optical Properties of Solids, eds. S. Nudelman and S.S. Mitra (Plenum, New York, 1966); Phys. Status Solidi 15 (1966) 627. [4] E.A. Davis and N.F. Mott, Phil. Mag. 22 (1970) 903. [5] S.K. AI-Ani and A.A. Higazy, J. Mat. Sci. 26 ( 1991 ) 3670. [6] S.K. AI-Ani, I.H. Al-Hassany and Z.T. AI-Dahan, J. Mat. Sci. (1994) in press. [7] F. Urbach, Phys. Rev. 92 (1953) 1324. [8] N.F. Mott and E.A. Davis, Electronic Processes in NonCrystalline Materials ( Clarendon Press, Oxford, 2nd Ed., 1979 ). [9] J. Tauc, Amorphous and Liquid Semiconductors (Plenum, London, New York, 1974).
ELSEVIER
1,
Optical Materials 5 (1996) 69-73
Optical properties of epoxy-glass microballoons composite Y. Ramadin a M. A1-Haj Abdallah a M. Ahmad S.K.J. A1-Ani b, S.G.K. A1-Ani b
a,
A. Zihlif a , * ,
a Physics Department, The Universi~ of Jordan, Amman, Jordan b Physics Department, Women Education College, Baghdad-Universi~, Baghdad, Iraq
Received 24 October 1994; accepted 4 July 1995
Abstract The optical properties in the uv-visible region of epoxy glass composite containing 0, 15, 35 and 55% by weight glass microballoons are reported. The obtained optical data are analysed in terms of absorption formula for non-crystalline materials. Assuming that non-direct transitions in the k-space are involved values of optical band gap (Eopt) were determined. From the Urbach edges, the width of the tall of localized states in the band gap (E) were evaluated at room temperature.
1. Introduction The epoxy-glass microballoons composite used in this study is a relatively new advanced material. It is based on addition of some light fillers as glass microballoons into polymer matrix. The obtained lighter structure is very useful in some specific advanced technology. The tensile mechanical properties and thermal behaviour (hot joints) of this composite were reported elsewhere [ 1 ]. The ac-conductivity and the dielectric properties in temperature range = 293-399 K in frequency range 100Hz-10kHz have been recently studied [2,3]. These properties showed frequency and temperature dependence. It is interesting to investigate the electronic and the optical transitions in such materials in terms of theories of non-crystalline solids. Therefore, this paper is concemed with the optical properties of the given composite in the uv-visible region for epoxy glass microballoons composite. * Corresponding author. 0925-3467/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDIO925-3467(95)00033-X
2. Experimental
2.1. Materials preparation
The glass microballoons were supplied by Shell company in U.S.A. The composite was prepared from the epoxy resin (Epon 828) cured by V-40 and hollow glass spheres of diameter ranges from 10 to 18 txm and typical effective density 0.21 g cm -3. Sheets of different glass microballoons concentration (0, 15, 35, and 55%) were obtained by mechanical mixing the epoxy resin, curing agent and the microballoons. The mixture was stirred carefully, and the test samples were allowed to react at 25°C for 24 hour and cured at 80°C in an oven for three hours. The final step was to postcure the composite at 150°C for two hours. Specimens were slowly cooled in the oven to the room temperature. Four disk-shaped specimens of 20 mm diameter were machined from the composite sheets for optical measurements.
70
Y. Ramadin et al. /Optical Materials 5 (1996) 69-73
o.o
8oo
~(2A)
~
g
.o
¢-.0
Wavelength (nm)
Fig. 1. Optical absorbance for composite specimens with different concentration of glass microballoons.
2.2. Optical measurements
a(co)hto= B ( h t o - Eopt)r,
The optical absorbance (A) is taken at a wavelength (A = 200-850 nm) using Pye-Unicam PU 8800 uvvisible spectrophotometer. A suitable cell was used to hold the specimen under investigation. After correcting for reflection at the first surface the absorption coefficient ( a ) at angular frequency (to) of radiation was calculated using absorption equation
I=Io exp( - at).
(1)
Hence a(to) = 2.303 log ~ = 2.303 A , t
1
t
where Io and I are the incident and transmitted intensity respectively. The actual sample thickness (t) was in the range (0.35 to 1.1) mm, and all measurements were performed at room temperature. 3. Results and discussion
3.1. High absorption At high absorption levels, where a(co) > 10 4 c m - 1, the absorption coefficient for non-crystalline materials has the following frequency dependence [ 3,4]
(2)
where B is a constant and r an exponent which can assume values of 1, 2, 3, 1/2 and 3/2 depending on the nature of the electronic transitions responsible for the optical absorption. Fig. 1 represents the spectra of the optical absorbance for 0, 15, 35 and 55% wt% glass microballoons composite samples while Fig. 2 shows the parameter (ahto) ~/r versus hto in accordance with Eq. (2) and for r = 2. The values of Eop t extrapolated from the linear portions in the high regions are listed in Table 1. A good straight line, however, is obtained with r = 2 (Fig. 2); for all samples and reasonable values of Eopt are obtained. This indicates that the transition energy for electrons is non-direct transitions in k-space. In a recent study on MgO-P205, glasses AI-Ani and Higazy [5,6] have reported two mechanisms for electron transitions for electrons namely, direct allowed (i.e. r = 1/2) transitions and non-direct (i.e. r = 2) transitions in k-space. The obtained results show that Eop=tends to decrease with the filler concentration as expected. The values of the constant B are obtained from the slope of the linear part of Fig. 2 and also listed in Table 1. It can be seen that the values of B are lower than those of the pure polymer and of the order of other glasses [6].
Y. Ramadin et al. / Optical Materials 5 (1996) 69-73
71
3.2. Low absorption
{dE) 112 o, (,eVe~' } i 18
In case of lower (in the range 1 c m - ' to 10 -4 c m - l ) absorption the absorption coefficient or(to) (range 1 cm - I to 104 c m - l ) , is described by Urbach formula [7]:
17 l{ 15
a( to) = Otoexp( hto/ AE),
14
(3)
where ao is a constant and AE is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap. Fig. 3 presents the Urbach plot for the samples in Fig. 1. The values of AE were obtained and also listed in Table 1. The exponential dependence of a(to) on hto for these samples indicates
1'3 12 II
5
I0
Ln~
9 8
7 6
5
4
Eg•
2 , , ,V
/J
-//
= 2,03 e V ]E --2.32 e V E
.- 2.53 eVz 3,5
3 2 ]
0 1.5
rE) 2
25
3
eV
Fig. 2. (aE)~/2 versus the incident photon energy for r = 2 (nondirect transition).
=o.]4ev (II),E 2 0 ]4eV
2.5 [
Table 1 Derived values of A E, the constant B and Eopt obtained from different optical mechanisms (Fig. 2) for four epoxy-glass microballoons samples Samples
Eop, (eV) (ahco) ~/2
Constant B (cm -~ e V - ' )
A E (eV)
0 wt pure: I 15 wt%: II 35 wt%: II1 55 wt%: IV
2.53 2,32 2.03 2.41
1193.4 306.3 190.4 818.0
0.34 0.24 0.60 0.55
/
151.5
Z'
2'5
(i,)~E • o s , v
( , v ~ : o ss.v
3'
eV . (E)
Fig. 3. Natural logarithem of ot versus the incident photon energy for r=2.
72
Y. Ramadin et al. / Ootical Material,~ 5 ( 19961 6Q-7~
Fig. 4. The SEM micrograph for specimens with (a) 5 and (b) 55 wt% glass microballoons.
that they obey Urbach's rule. The values of AE were higher than those reported for non-crystalline materials [8,9]. Those tails become higher as the concentration of the glass balloons increases which is consistance with Eopt variation. The decrease in Eop, with glass content can be understood by considering the mobility gap variation in composites as proposed by Davis and Mott [4]. Since Eop, represents generally the energy difference between the localized states in the valence band and extended states in the conduction band or vice versa, then we assume AEc = AEv. Hence, Eopt + AE represents the mobility gap. However the used composite is a heterogenous material where the electrical conduction mainly depends on structural defects, voids
and impurities which probably makes the Eoptnarrower by increasing the width of band tailing.
3.3. Compositemorphology The SEM micrograph for 5 and 55 wt% glass microballoons composite samples (Fig. 4) display the textural and morphological evolution of the composite as a function of filler content. It is clear that at low concentration, the microballoons are distributed randomly in the matrix with no surface contacts between them, while at high concentration some contacts are existing between the adjacent glass microballoons. A small number of glass microballoons are pulled out from
Y. Ramadin et al. /Optical Materials 5 (1996) 69-73
fractured surfaces which appear with dark spheres through the epoxy matrix. Other features of the composite morphology can also be drawn from the micrographs as the hetrogeneous distribution due to mechanical mixing of glass microballoons with the epoxy resin. The thermal curing may displace the microballoons to stick to each other, especially at high filler concentration. Certainly, this composite morphology plays a rather effective role in the optical absorption process.
4. Conclusion The behaviour of the optical absorption in the uvvisible region for epoxy-glass microballoons composite is studied as a function of filler concentration at room temperature. Two formulae were used to analyse the obtained optical data in high and low absorption ranges. The energy gaps were evaluated in both optical absorption cases.
73
Acknowledgements The authors thank Professor R.J. Farris, and L. Feldman of Umass, Amherst, U.S.A. for their contribution in the composite preparation.
References [ 1 ] A. Zihlif, L. Feldman and R. Farris, J. Mat. Sci. 24 ( 1989 ) 3267. [2] M. Shahin, M. AI-Haj Abdallah, A. Zihlif and R. Farris, Dielectric Properties of Epoxy-Glass Microballoons Composite, Polymer Int., submitted for publication; M. Shahin, M. Sc. thesis, Physics Dept. University of Jordan (1994). [ 3 ] J. Tauc, Optical Properties of Solids, eds. S. Nudelman and S.S. Mitra (Plenum, New York, 1966); Phys. Status Solidi 15 (1966) 627. [4] E.A. Davis and N.F. Mott, Phil. Mag. 22 (1970) 903. [5] S.K. AI-Ani and A.A. Higazy, J. Mat. Sci. 26 ( 1991 ) 3670. [6] S.K. AI-Ani, I.H. Al-Hassany and Z.T. AI-Dahan, J. Mat. Sci. (1994) in press. [7] F. Urbach, Phys. Rev. 92 (1953) 1324. [8] N.F. Mott and E.A. Davis, Electronic Processes in NonCrystalline Materials ( Clarendon Press, Oxford, 2nd Ed., 1979 ). [9] J. Tauc, Amorphous and Liquid Semiconductors (Plenum, London, New York, 1974).