Materials Letters 59 (2005) 1125 – 1129 www.elsevier.com/locate/matlet
Effects of different amount of As2O3 and TiO2 on the chemical, physical and electrical properties of the base material MnO2 Kadir Esmera, Umit Kadiroglub, Erdogan Tarcana, Asgar Kayanb,* a Kocaeli University, Department of Physics, 41300, Izmit, Turkey Kocaeli University, Department of Chemistry, 41300, Izmit, Turkey
b
Received 7 September 2004; received in revised form 18 November 2004; accepted 27 November 2004 Available online 4 January 2005
Abstract MnO2 oxide systems containing different weight percent of TiO2 and As2O3 were prepared. A number of studies, i.e., Fourier Transform Infrared, X-ray powder diffraction, electrical conductivity and dielectric properties on these mixed oxide systems, were carried out. These investigations show that the doping amount of As2O3 and TiO2 influences structure, conductance, electrical and dielectric behavior of MnO2 system. The optical investigations show that MnO2 reacts with As2O3 to form Mn2As2O7 and Mn2O3. TiO2 appears in the form of rutile phase. Dielectric constants eV and eU of 5 wt.% As2O3-doped composite were calculated to be ~30 and ~38 at 13.5 kHz, respectively. The formation of Mn2As2O7, Mn2O3 and the presence of TiO2 rutile phase and the degree of polarization or charge displacement depending on frequency played an effective role in the magnitude of eV, eU and r. D 2004 Elsevier B.V. All rights reserved. Keywords: Manganese (IV) oxide; Rutile phase; Mixed oxide systems; Physical and chemical properties
1. Introduction There is currently great interest in the technological properties of composites because of the demands for various application fields such as catalysts, capacitors, thermistors, transducers, fuses sonar generators, electrical insulators [1– 5]. The mixed oxide systems such as ZrO2–CeO2–Y2O3 [6– 8], ZrO2–TiO2–Y2O3 [9], Al2O3–TiO2 [10], (Pb, Nb )–TiO2 [11], (Ba, Bi, Nb )–TiO2 [12], (Cr, Nb)–TiO2 [13], (Ca, Ta)– TiO2 [14], Sb2O3–As2O3–alkali halides [15], etc., have been studied in detail. Interesting studies on As2O3 mixed with different semiconducting oxides such as SiO2, GeO2 and V2O3 are also available in the literature [16–18]. In particular, MnO2 as doped material is an interesting one since it exists in different valence oxidation state in different mixed oxide system [19]. The MnO2 was considered as base
* Corresponding author. Tel.: +90 262 324 99 10/264; fax: +90 262 324 99 09. E-mail address:
[email protected] (A. Kayan). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.037
material in this study. Any study on Mn–Ti–As mixed oxide systems has not been reported yet. The effects of arsenic (III) oxide and titanium (IV) oxide on the MnO2-based composite were not investigated either. The purpose of this study is to characterize the structure of composites and also to understand the electrical and dielectric properties of the Mn–Ti–As mixed oxide mixture over a wide range of frequencies at room temperature.
2. Experimental procedure In this study, the solid–solid reaction was chosen though there are a number of preparative methods such as crystallization of solutions, vapor phase transport and electrochemical reduction methods. This route was mainly chosen due to its simplicity and suitability to starting materials in order to obtain thin films and single crystals. Manganese (IV) oxide (MnO2, 90%, Aldrich), titanium (IV) oxide (TiO2, 99.9%, Aldrich) and arsenic (III) oxide (As2O3, 99%, Fluka) were used as received. Composites were
K. Esmer et al. / Materials Letters 59 (2005) 1125–1129
prepared as 80 wt.% MnO2, 10 wt.% TiO2, 10 wt.% As2O3 and 80 wt.% MnO2, 15 wt.% TiO2 and 5 wt.% As2O3, respectively. The oxide mixtures were firstly stirred by a magnetic stirrer for 3 h. The Mn–Ti–As oxide mixtures were kept in an oven at 700 8C for 24 h in air and then cooled slowly to room temperature (25 8C). The melting points of MnO2 (535 8C) and As2O3 (315 8C) were below the chosen temperature. Thus these compounds will be well melted and also any impurities would be removed within those compounds. Although the melting point of TiO2 (1855 8C) is higher than the chosen temperature, it was intended to investigate the diffusion of TiO2 within the composite. The infrared spectra were taken between 400 and 2000 cm1 using a Shimadzu 8201/86601 PC spectrometer. The Mn–Ti–As oxide powders were mixed with KBr and pressed at 7 tons to form pellets which have diameter of 13 mm. The vibrational measurements were performed using these pellets. For dc conductivity and capacitive measurements, a new set of pellets was prepared by pressing the Mn–Ti–As oxide systems at 7 tons having an area of 4.96101 cm2 and thickness of 9.1102 cm. The both faces of resulting pellets were coated with aluminum under high vacuum atmosphere (~106 torr). The coating was done using a Univex 300 Leybold vacuum apparatus. The dc conductivity was measured using a Keithley 2400 Source-meter. The capacitive measurements were performed in the frequency range 1 kHz to 5 MHz and in the voltage range 0–5 V with an Impedance/Gain-Phase Analyzer HP 4194A. X-ray spectra were taken using an XRD Rigaku DMAX 2200. The X-ray wavelength was CuKa radiation, k=1.5405 2. All measurements were taken at room temperature.
3. Results and discussion 3.1. FTIR and XRD measurements The FTIR spectra of the pure MnO2 (base material) and the composite having 80 wt.% MnO2, 10 wt.% TiO2 and 10 wt.% As2O3 are shown in Fig. 1. The FTIR spectrum of pure crystalline MnO2 shows double degenerate vibrations at 660–670 cm1 and strong peaks at 530 and 608 cm1. FTIR spectrum of pure As2O3 exhibits four fundamental absorption bands at 1050 cm1 (symmetric stretching vibrations), 618 cm1 (symmetric bending vibrations), 795 cm1 (doubly degenerate stretching vibrations) and 505 cm1 (doubly degenerate bending vibrations) [19]. Rutile phase of TiO2 exhibits strong absorption bands in the region of 800–600 cm1 [20]. Because of the structural change in MnO2, after reacting with As2O3 and TiO2, an intense peak appears at 655 cm1 in the FTIR spectrum of composite. An improvement in the region between 650 and 600 cm1 was noted suggesting
96.0 95.5 95.0 94.5
wt. 10% As2O3 wt. 10% TiO2 Composite
94.0 93.5 93.0
% Transmittance
1126
92.5
Pure MnO2
92.0 91.5 91.0 90.5 90.0 89.5 89.0 88.5 88.0 87.5 87.0 2000.0 1900.0 1800.0 1700.0 1600.0 1500.0 1400.0 1300.0 1200.0 1100.0 1000.0 900.0 800.0 700.0 600.0 500.0 -1
400
Wavenumber / cm
Fig. 1. FTIR spectra of composite (80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3) and pure MnO2.
that the peak at 608 cm1 of MnO2 and the peak at 618 cm1of As2O3 overlapped with the peaks of TiO2. In the FTIR spectrum (Fig. 1) of composite, bands due to vibrations at 1050 and 795 cm1 of As2O3 are observed to be shifted to 990 and 850 cm1, respectively. This can be interpreted as result of the solid–solid interactions at 700 8C. As2O3 gets incorporated into the structure of MnO2 via oxygen bridges to form Mn2As2O7 which was confirmed with XRD observations. The composite consisting of 80 wt.% MnO2, 15 wt.% TiO2 and 5 wt.% As2O3 gave almost the same FTIR spectrum. However, the intensities of peaks were different because the composite had different amount of components. These results are consistent with the XRD results. XRD patterns (Figs. 2 and 3) of both composites confirm the formation of Mn2As2O7 (JCPDS Files No. 44-0076) and Mn2O3 (bixbyite-C, JCPDS Files No. 411442) and also show the presence of TiO2 rutile phase (JCPDS Files No. 21-1276). The results were summarized in Tables 1 and 2. The XRD observations reveal an information about the reactivity of As2O3 towards MnO2. As can be seen in Figs. 2 and 3, no diffraction lines due to As2O3 and compounds between TiO2 and MnO2 are observed. With increase in the amount of As2O3 from 5 wt.% to 10 wt.%, an increase in the intensity of the lines at d=4.3667, 3.1974 and 3.0214 2 due to Mn2As2O7 can be seen (Tables 1 and 2). Weak lines due to Mn2As2O7 and intense lines due to TiO2 rutile phase overlap at d=3.2384 and 1.6852 2 (Table 1). From Tables 1 and 2, it can be noted that some values of d, 2h and intensity are the same because of overlapping the lines of Mn2As2O7, TiO2 and also Mn2O3. Although some of the TiO2 had been expected to diffuse into the structure of MnO2, XRD results did not support this assumption.
K. Esmer et al. / Materials Letters 59 (2005) 1125–1129
XRD Intensity (a.u)
Table 1 XRD data for the composite containing 80 wt.% MnO2 +15 wt.% TiO2+5 wt.% As2O3
b
800
80 wt.% MnO2+15 wt.% TiO2+ 5 wt.% As2O3
600
Compound
2h
d-Value (2)
Intensity
I/I 0 (%)
Mn2As2O7 (a)
27.520 27.900 29.460 34.080 54.400 59.000 23.180 32.980 38.300 45.180 49.340 55.240 60.660 64.160 65.980 67.600 27.520 36.140 39.260 41.300 44.120 54.400 56.700 62.840 64.160 69.080 69.860
3.2384 3.1952 3.0294 2.6286 1.6852 1.5643 3.8340 2.7137 2.3481 2.0052 1.8455 1.6615 1.5254 1.4503 1.4147 1.3847 3.2384 2.4833 2.2929 2.1842 2.0509 1.6852 1.6221 1.4776 1.4503 1.3586 1.3453
356 107 42 29 163 23 94 422 96 45 60 113 22 47 58 22 356 138 33 64 34 163 53 31 47 56 29
86 26 10 8 40 6 24 100 24 12 16 28 6 12 14 6 86 34 8 16 8 40 14 8 12 14 8
a,c 400
a b
200
b c
a
b
a
a
c
b
b
a,c b
b
c
Mn2O3 (b)
c
0
10
20
30
40
50
60
70
2θ Fig. 2. XRD patterns of composite (80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3): (a) lines due to Mn2As2O7; (b) lines due to Mn2O3; (c) lines due to TiO2 rutile phase. TiO2 (c)
3.2. Electrical conductivity measurements Conductivities (r) were calculated using resistivity (q) values with the following equations: q ¼ ½ðV =d Þ=ði=AÞ
ðXcmÞ
ð1Þ
and r ¼ 1=q
X1 cm1
ð2Þ
where V is the applied voltage, d is the distance between electrodes, i is the current and A is the area of pellet as a unit-section. Dc current–voltage plots of composites prepared in various amount of TiO2 and As2O3 are shown in Fig. 4. It shows that As2O3 has significant impact on the conductivity of MnO2. The conductivity was higher for 5 wt.% As2O3doped composite than that of 10 wt.% As2O3-doped composite. In other words, the conductivity increases as the doping amount of TiO2 is increased from 10 wt.% to
Table 2 XRD data for the composite containing 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3 Compound
2h
d-Value (2)
Intensity
I/I 0 (%)
Mn2As2O7 (a)
20.320 27.480 27.880 29.540 34.040 39.220 41.300 42.660 54.380 23.100 32.940 38.240 45.180 49.380 55.160 64.120 65.780 67.460 27.480 36.140 39.220 41.300 44.080 54.380 56.680 64.120 69.080 69.880
4.3667 3.2431 3.1974 3.0214 2.6316 2.2951 2.1842 2.1177 1.6857 3.8471 2.7169 2.3517 2.0052 1.8441 1.6637 1.4512 1.4185 1.3872 3.2431 2.4833 2.2951 2.1842 2.0527 1.6857 1.6227 1.4512 1.3586 1.3450
32 244 162 71 38 40 42 29 134 77 481 102 56 61 155 41 65 31 244 73 40 42 23 134 29 41 50 20
8 52 34 16 8 10 10 6 28 16 100 22 12 14 34 10 14 8 52 16 10 10 6 28 6 10 12 6
b
XRD Intensity (a.u)
800
80 wt.% MnO2+10 wt.% TiO2+ 10 wt.% As2O3
Mn2O3 (b)
600
a,c
400
a 200
b
a
a
b c b a
c
b
b
a,c b
b,c b c
c
0
10
20
30
40
50
60
1127
70
2θ Fig. 3. XRD patterns of composite (80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3 ): (a) lines due to Mn2As2O7; (b) lines due to Mn2O3; (c) lines due to TiO2 rutile phase.
TiO2 (c)
1128
K. Esmer et al. / Materials Letters 59 (2005) 1125–1129 0,2 0,18
3 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3
log(ε'')
0,16 0,14
I (mA)
80 wt.% MnO2+15 wt.% TiO2+5wt.% As2O3 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3
2,5 2 1,5
0,12
1
0,1
0,5 0
0,08
2
3
4
0,06 0,04
6
7
Fig. 6. Variation of logarithmic eU with logarithmic frequency (Hz); (D) 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3, (5) 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3.
0,02 0 0,00
5
log (f/Hz)
0,50
1,00
1,50
2,00
2,50
V (Volt) Fig. 4. Current versus voltage for composites of (D) 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3, (5) 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3.
15 wt.% TiO2. These changes can also be explained the cross linking of a part of AsO3 units with Mn4+ ions to form As–O–Mn bonds [19]. When the amount of As2O3 is increased from 5 wt.% to 10 wt.%, the amount of the formation of cross links increased. This is confirmed by the XRD patterns which include no diffraction lines due to the As2O3. In addition, an increase in the intensity of lines due to Mn2As2O7 supports the formation of crosslinks. Thus, these crosslinks decrease the polarizability leading to an increase in the values of conductivity. The reduction of MnO2 (Mn4+, d3 configuration) to Mn2O3 (Mn3+, d4) by heating at 700 8C also effects the conductivity. Conductivity (r) corresponds to 1.4 V was calculated as 4.7107 V1 cm1, 1.5108 V1 cm1 and 9.2109 V1 cm1 for pure MnO2, composite of 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 and composite of 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3, respectively. As seen from calculated values, the conductivity becomes lower than that of pure MnO2 which has IV oxidation state (or d3 configuration). In the study of the previous paper, the conductivity values of
SnO-doped material were found higher compared to the conductivity of TiO2-doped material [21]. 3.3. Dielectrical properties The real part of dielectric constant (relative permittivity), eV, is a measure of the capacitance whereas the imaginary part of dielectric constant, eU, is a measure of conductance. Dielectric constant (eV and eU) and loss factor (tan d) were calculated with the contribution of the following equations: C ¼ eVe0 A=d
ð3Þ
tand ¼ eW=eV
ð4Þ
and eW ¼ r=xe0 A=d
ð5Þ
(C: measured capacitance, e 0: space constant: 8.851012 F/m and x: angular frequency, r: conductance). The plots of eV versus frequency, eU versus eV, eU versus frequency and tan d versus frequency are shown in Figs. 5– 8. The variations corresponding to 1 kHz to 5 MHz frequencies are interpreted as the following: The real part of dielectric constant, eV, of these composites gradually 60
80 wt.% MnO2+10 wt.%TiO2+10 wt.% As2O3 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3
35 50
10 KHz
30 40
ε''
25 20
ε'
20
15 10 5 0 1,0E+03
5 MHz
30
80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 10
80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3
0 0
1,0E+06
2,0E+06
3,0E+06
4,0E+06
5,0E+06
6,0E+06
f (Hz) Fig. 5. Variation of real part of dielectric constant, eV, with frequency (Hz); (w) 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3, (5) 80 wt.% MnO2+15 wt.%TiO2+5 wt.% As2O3.
10
20
30
40
50
60
ε' Fig. 7. The complex plot of dielectric constant (imaginary part(eU) versus real part (eV)); (w) 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3, (5) 80 wt.% MnO2+15 wt.% TiO2+ 5 wt.% As2O3. Dash lines at high frequency region show the simulation for the relaxation.
K. Esmer et al. / Materials Letters 59 (2005) 1125–1129 7 6
80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3
tan δ
5 4 3 2 1 0 2
3
4
5
6
7
log (f/Hz) Fig. 8. Variation of loss factor, tan d with logarithmic frequency, log ( f/Hz).
decreased with an increase in the frequency values (Fig. 5). Fig. 5 clearly reveals that eV values of composite 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 measured in 1 kHz to 5 MHz regions is larger than eV values of 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3. This demonstrates that adding 10 wt.% As2O3 decreases the polarizability in the composite when compared to 5 wt.% As2O3. When eV was measured for pure MnO2, the values change from 60 to 50 in the frequency region 1 kHz to 5MHz [20]. Generally, eV changes in the range of 25–170 for different ceramic composite materials [22]. The imaginary part of dielectric constant, eU, of 5 wt.% As2O3-doped composite indicates that eU is around 38 in low frequency region, decreases gradually with frequency up to 650 kHz (eU=3.6) (Fig. 6). The drop in eU values in high frequency region can correspond to small space charge polarization. This is reasonable because the space charge polarization effect relaxed out in composite at frequencies about 106 Hz. This is consistent with the literature about doped materials [19]. The eU of 10 wt.% As2O3-doped composite demonstrated similar behavior. Both eV and eU values for the Mn–Ti–As oxide mixture vary significantly with frequency in 1 kHz to 0.5 MHz regions. The plot of eU versus eV is displayed in Fig. 7 which shows a partially resolved semicircle at high frequencies and a constant-phase element corresponding to capacitive components. The semicircle part demonstrates that two main types of relaxation are found in this material, corresponding to dipole moments, related to polarization, caused by association of substitutional oxides (As2O3 and TiO2). The composites possess low loss factor as seen in Fig. 8 over a range of frequencies covering 1 kHz to 5 MHz regions. Since energy loss is proportional to e" and energy storage to eV [23]. Fig. 8 shows that the energy losses decrease at high frequency region and increase at low frequency region.
4. Conclusion In summary, composites having 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 and 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3 were prepared by solid–solid reaction at 700 8C and characterized by FTIR and XRD measurements. The optical measurements showed that the formation
1129
of Mn2As2O7 and Mn2O3 (bixbyite-C) and the presence of TiO2 rutile phase. The formation of these compounds and the presence of TiO2 affected the conductivity, electrical and dielectrical properties compared to those of base material MnO2. Different weight percentage was also effective in the properties mentioned above. For instance, the first composite 80 wt.% MnO2+15 wt.% TiO2+5 wt.% As2O3 had higher conductivity than the second one 80 wt.% MnO2+10 wt.% TiO2+10 wt.% As2O3. The magnitude of eV, eU and tan d changed depending on frequency.
Acknowledgements This work was supported by the Research Foundation of Kocaeli University, Project No.: 2002/57.
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