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Plasma-sprayed cordierite: Dielectric and electrical properties
Surface and Coatings Technology, 37 (1989) 297
-
303
297
PLASMA-SPRAYED CORDIERITE: DIELECTRIC AND ELECTRICAL PROPERTIES* H. G. WANG and H. HERMAN
Department of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275 (U.S.A.) (Received September 1, 1988)
Summary The dielectric and electrical properties of plasma-sprayed deposits of cordierite were studied. It was found that both the dielectric constant and the electrical conductivity increase with temperature. The dielectric constant of the as-sprayed deposit is less temperature dependent than that of the annealed deposit. The activation energies for the conductivity measured at different frequencies were found to be higher for the as-sprayed deposit.
1. Introduction Cordierite (2MgO—2A1203—5SiO2) is an oxide having a low dielectric constant, high electrical resistivity and high chemical durability. Consequently, the oxide has been used as substrates and insulators for integrated circuits [1, 2]. In a recent study, cordierite deposits were formed by plasma spraying, and the structure and phase transformations of the deposits were examined using differential thermal analysis and X-ray diffractometry [3]. The assprayed deposit was found to be amorphous, with a transformation to highcordierite occurring at about 1000 °C. The high-cordierite structure can be obtained by annealing the as-sprayed deposit above 1000 °C. Dielectric behavior of plasma-sprayed cordierite has been studied as a function of frequency at room temperature [4]. The dielectric constant of as-sprayed amorphous cordierite is about 7.0, which is essentially independent of frequency. The dielectric constant of the annealed high-cordierite deposit varies from 7.3 at 100 Hz to 5.7 at 106 Hz, which is caused by spacecharge polarization at low frequencies. Dielectric loss of the as-sprayed amorphous deposit is greater than that of annealed cordierite over the frequency range froma migration 102 to iO~Hz. Thetogradual increase in dielectric loss 5 Hz reveals loss due ion jumping. below i0 *paper presented at NTSC 88, the National Thermal Spray Conference, Cincinnati, OH, U.S.A., October 23 0257-8972/89/$3.50
-
27, 1988.
©
Elsevier Sequoia/Printed in The Netherlands
298
The aim of the present study is to evaluate dielectric and electrical properties as a function of temperature and to determine the activation energies for conduction for both the as-sprayed and the annealed deposits. Moreover, the relation between the conductivities of the deposits at high temperatures and their microstructure will be examined. 2. Experimental procedure The powder used in this investigation was fused, cast and comminuted cordierite, which was supplied by Muscle Shoals Minerals (Tuscumbia, AL, U.S.A.). The powder was characterized using a light scattering technique, yielding a mean particle size of 43 pm with a standard deviation of 28 pm [4]. The deposits were formed using an automated plasma system (PlasmaTechnik AG., Switzerland). After spraying, the deposits were removed from salt-coated steel substrates by submersion in water. The nominal oxide chemistry and the plasma spray parameters are given elsewhere [3]. The annealing treatment of the as-sprayed deposit was carried out in an electric air furnace at 1100 °Cfor 12 h. Measurements of the dielectric constant and dielectric loss as a function of temperature were performed on a capacitance bridge (General Radio model 1615A) at a frequency of 10 kHz. The deposit was polished on both sides, to yield a specimen of dimensions 25 mm X 25 mm X 0.5 mm. Subsequently, the two sample surfaces were painted with conductive silver paint (Dupont 4517) and baked at 100 °Cfor 2 h to remove volatile organics from the paint. The sample was inserted between two eyelet connectors of a coaxial cable and then placed in a circular furnace. The heating cycle ranged between room temperature and 500 °C,with the heating rate controlled to about 5 °Cmin1. The resistivity as a function of temperature was measured using a twopoint method at a constant voltage of 5000 V and a frequency of 60 Hz. The sample was ultrasonically cleaned in an acetone solution prior to measurement. The sample was clamped in a pair of stainless steel electrodes, and placed inside a quartz tube within the circular furnace. The heating rate was the same as that used for the dielectric measurement. The maximum temperature of the thermal cycling was 700 °C. For the transmission electron microscopy study, the deposits were mechanically polished down to about 70 pm and then ion beam thinned at an incident angle of 12°to the surface for about 10 h. This was followed by the deposition of a thin layer of carbon before examination under a Philips CM-12 electron microscope equipped with a double-tilting goniometer and operated at 120 kV. 3. Results and discussion The dielectric constant as a function of temperature, shown in Fig. 1, increases only slowly over the testing temperature range for the as-sprayed
299 6
0.4
ç
Anneoled
I:
~
—
00
~rayed
200
300
400
Temperature (SC)
500
600
0.0
100
200
300
400
500
600
Temperature (C)
Fig. 1. Variation in dielectric constant K’ with temperature at a frequency of 10 kHz. Fig. 2. Variation in dielectric loss K” with temperature at a frequency of 10 kHz.
deposit, until 475 °Cwhere it increases. Although the annealed deposit has a lower dielectric constant than that of the as-sprayed deposit below 400 °C, it exhibits a significant increase in dielectric constant above 400 °C. Consequently, the dielectric constants of the annealed deposit are about 50% higher than those of the as-sprayed deposit for temperatures above 450 °C.The dielectric loss vs. temperature curves shown in Fig. 2 exhibit minima for both the as-sprayed and the annealed deposits at 350 °Cand 150 °Crespectively. It is believed that the network of a vitreous system is rather loose while the ion groups in a crystalline lattice are structured more rigidly [5]. Therefore, the dispersion of ion groups is more difficult in crystals. It has been reported that as-sprayed cordierite is amorphous, becoming hexagonal on annealing [3]. Consequently, both the dielectric constant and the dielectric loss display a low temperature range where the values for the as-sprayed deposit are higher than those for the annealed deposit. The strong increase in dielectric constant of the annealed deposit at high temperatures is attributed to enhanced space change polarization caused by crystal defects [6]. The concave shape of the curves of the dielectric loss vs. temperature is attributed to moisture and migration loss effects, since the deposits contain pores which absorb moisture. As a result, the conductivity is increased by the absorption of water, which vaporizes with increased temperature, in turn decreasing the conductivity and dielectric loss. At higher temperatures, migration losses dominate. The relationship between the conductivity and the dielectric properties can be written as [7] o=2irfK’K”K0 (1) where a is the electrical conductivity, f is the frequency, K’ is the dielectric constant, K” is the dielectric loss and K0 is the vacuum dielectric constant. On the basis of eqn. (1) the conductivities of the as-sprayed and annealed deposits were calculated for the temperature range where the dielectric losses
300 21
24
.20
22
~
As-sprayed
:
20
~
3/T(K)
10
8
Annealed
l~ 103/T(K)
l~
6
Fig. 3. Logarithm of the resistivity as a function of reciprocal temperature at a frequency of 10 kHz. Fig. 4. Logarithm of the resistivity as a function of reciprocal temperature at a frequency of 60 Hz and a constant voltage of 5 kV.
increase with temperature. The decrease in resistivity with temperature points to an increase in the concentration of charge carriers, such as ion groups. An activation energy (i.e. the energy required to free the charge carriers from neighboring ions and the transport of these charge carriers) can be evaluated using an Arrhenius-type equation [8]
o
I
=
E\
O~exp~j——)
(2)
where O~ is a pre-exponential constant, E is the activation energy for electrical conduction, k is Boltzmann’s constant and T is the absolute temperature. The logarithm of the resistivity is plotted vs. the reciprocal temperature in Fig. 3. Using eqns. (1) and (2), the activation energies for conduction are found to be 0.66 ±0.12 eV for the as-sprayed deposit and 0.42 ±0.05 eV for the annealed deposit. The electrical resistivity measurements are evaluated for activation energies in Fig. 4 using eqn. (2), yielding 1.48 ±0.05 eV and 1.13 ±0.03 eV for the as-sprayed and annealed deposits respectively. Both dielectric and electrical measurements show that the activation energy of the as-sprayed deposit is significantly greater than that for the annealed deposit. This can be attributed to different mobilities of the charge carriers for the different microstructures. Figure 5 is a bright field image of the as-sprayed deposit showing a homogeneous structure with an amorphous halo ring in the corresponding selected area diffraction pattern. A bright field image of the annealed deposit, shown in Fig. 6, displays a crystalline defected structure, such as the dislocation network indicated by the arrows, as well as high angle grain boundaries. A strongly diffracting grain, labelled A, has a plane normal to the [1211] direction. Fine-scale crystalline defects can be observed from a two-dimensional lattice image (Fig. 7), which was taken
301
Fig. 5. Transmission electron micrograph of the as-sprayed deposit showing a low contrast image of amorphous and halo rings in a selected area diffraction pattern (upper left).
Fig. 6. Transmission electron micrograph of the annealed deposit (1100 °C for 12 h) showing a dislocation network as indicated by the arrows and a strongly diffracting grain in the [1211] zone direction.
from an equal thickness region. Some fringes are terminated by a cluster of vacancies, as marked by V. Some fringes are distorted by dislocations, as exemplified by D. Generally, the mobility of the charge carriers is enhanced through the crystal defects at elevated temperatures, resulting in the higher electrical conductivity and the lower activation energy for the annealed deposit. The difference in the activation energies obtained by the dielectric and electrical measurements is due to the difference in frequency. The former was measured at 10 kHz and the latter at 60 Hz. The influence of frequency on the resistivity and the activation energy for conduction has also been
302
~-
‘~8151~—
Fig. 7. A (0001) two-dimensional lattice image from the annealed deposit (1100 °Cfor 12 h) showing several defects marked by the white arrows and a fringe spacing of 8.5 A corresponding to the plane spacing of (1120).
observed for a synthetic quartz crystal. The conductivities measured at high frequencies were greater than those measured at low frequencies [9]. 4. Conclusions The dielectric and electrical properties of plasma-sprayed cordierite were studied. The dielectric constant is less temperature dependent for the as-sprayed deposit than for the annealed deposit. At low temperatures, the dielectric constant of the as-sprayed deposit is higher than that of the annealed deposit, which is attributed to the difference in dispersion of ions in different crystal structures. The moisture absorbed by pores contributes to a high dielectric loss at low temperatures. The activation energies for conduction were evaluated at the different frequencies, with the activation energy for conduction being higher for the as-sprayed deposit than for the annealed deposit. This effect is attributed to increased mobility of charge carriers through the observed defects in crystalline structure at elevated temperatures. Acknowledgment The authors are grateful to V. Ben for his assistance in the resistivity measurement and to Y. Zhu for help in the high resolution transmission electron microscopy work. References 1 K. Watanabe and E. A. Giess, J. Am. Ceram. Soc., 68 (4) (1985) C-102. 2 H. S. Kanost, Interceram, 28 (1) (1967) 61.
303 3 H. G. Wang, G. S. Fischman and H. Herman, Plasma sprayed cordierite: structure and phase transformations, to be published in J. Mater. Sci., (1989). 4 L. Brown, The dielectric behavior of plasma-sprayed oxides, Ph.D. Thesis, State University of New York at Stony Brook, 1987. 5 J. M. Stevels, J. Non-C ryst. Solids, 73 (1985) 165. 6 K. V. Rao and A. Smakula, J. Appl. Phys., 36 (6) (1965) 2031. 7 W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, p. 937. 8 W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, p. 875. 9 A. De and K. V. Rao, J. Mater. Sci., 23 (1988) 661.
-
303
297
PLASMA-SPRAYED CORDIERITE: DIELECTRIC AND ELECTRICAL PROPERTIES* H. G. WANG and H. HERMAN
Department of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275 (U.S.A.) (Received September 1, 1988)
Summary The dielectric and electrical properties of plasma-sprayed deposits of cordierite were studied. It was found that both the dielectric constant and the electrical conductivity increase with temperature. The dielectric constant of the as-sprayed deposit is less temperature dependent than that of the annealed deposit. The activation energies for the conductivity measured at different frequencies were found to be higher for the as-sprayed deposit.
1. Introduction Cordierite (2MgO—2A1203—5SiO2) is an oxide having a low dielectric constant, high electrical resistivity and high chemical durability. Consequently, the oxide has been used as substrates and insulators for integrated circuits [1, 2]. In a recent study, cordierite deposits were formed by plasma spraying, and the structure and phase transformations of the deposits were examined using differential thermal analysis and X-ray diffractometry [3]. The assprayed deposit was found to be amorphous, with a transformation to highcordierite occurring at about 1000 °C. The high-cordierite structure can be obtained by annealing the as-sprayed deposit above 1000 °C. Dielectric behavior of plasma-sprayed cordierite has been studied as a function of frequency at room temperature [4]. The dielectric constant of as-sprayed amorphous cordierite is about 7.0, which is essentially independent of frequency. The dielectric constant of the annealed high-cordierite deposit varies from 7.3 at 100 Hz to 5.7 at 106 Hz, which is caused by spacecharge polarization at low frequencies. Dielectric loss of the as-sprayed amorphous deposit is greater than that of annealed cordierite over the frequency range froma migration 102 to iO~Hz. Thetogradual increase in dielectric loss 5 Hz reveals loss due ion jumping. below i0 *paper presented at NTSC 88, the National Thermal Spray Conference, Cincinnati, OH, U.S.A., October 23 0257-8972/89/$3.50
-
27, 1988.
©
Elsevier Sequoia/Printed in The Netherlands
298
The aim of the present study is to evaluate dielectric and electrical properties as a function of temperature and to determine the activation energies for conduction for both the as-sprayed and the annealed deposits. Moreover, the relation between the conductivities of the deposits at high temperatures and their microstructure will be examined. 2. Experimental procedure The powder used in this investigation was fused, cast and comminuted cordierite, which was supplied by Muscle Shoals Minerals (Tuscumbia, AL, U.S.A.). The powder was characterized using a light scattering technique, yielding a mean particle size of 43 pm with a standard deviation of 28 pm [4]. The deposits were formed using an automated plasma system (PlasmaTechnik AG., Switzerland). After spraying, the deposits were removed from salt-coated steel substrates by submersion in water. The nominal oxide chemistry and the plasma spray parameters are given elsewhere [3]. The annealing treatment of the as-sprayed deposit was carried out in an electric air furnace at 1100 °Cfor 12 h. Measurements of the dielectric constant and dielectric loss as a function of temperature were performed on a capacitance bridge (General Radio model 1615A) at a frequency of 10 kHz. The deposit was polished on both sides, to yield a specimen of dimensions 25 mm X 25 mm X 0.5 mm. Subsequently, the two sample surfaces were painted with conductive silver paint (Dupont 4517) and baked at 100 °Cfor 2 h to remove volatile organics from the paint. The sample was inserted between two eyelet connectors of a coaxial cable and then placed in a circular furnace. The heating cycle ranged between room temperature and 500 °C,with the heating rate controlled to about 5 °Cmin1. The resistivity as a function of temperature was measured using a twopoint method at a constant voltage of 5000 V and a frequency of 60 Hz. The sample was ultrasonically cleaned in an acetone solution prior to measurement. The sample was clamped in a pair of stainless steel electrodes, and placed inside a quartz tube within the circular furnace. The heating rate was the same as that used for the dielectric measurement. The maximum temperature of the thermal cycling was 700 °C. For the transmission electron microscopy study, the deposits were mechanically polished down to about 70 pm and then ion beam thinned at an incident angle of 12°to the surface for about 10 h. This was followed by the deposition of a thin layer of carbon before examination under a Philips CM-12 electron microscope equipped with a double-tilting goniometer and operated at 120 kV. 3. Results and discussion The dielectric constant as a function of temperature, shown in Fig. 1, increases only slowly over the testing temperature range for the as-sprayed
299 6
0.4
ç
Anneoled
I:
~
—
00
~rayed
200
300
400
Temperature (SC)
500
600
0.0
100
200
300
400
500
600
Temperature (C)
Fig. 1. Variation in dielectric constant K’ with temperature at a frequency of 10 kHz. Fig. 2. Variation in dielectric loss K” with temperature at a frequency of 10 kHz.
deposit, until 475 °Cwhere it increases. Although the annealed deposit has a lower dielectric constant than that of the as-sprayed deposit below 400 °C, it exhibits a significant increase in dielectric constant above 400 °C. Consequently, the dielectric constants of the annealed deposit are about 50% higher than those of the as-sprayed deposit for temperatures above 450 °C.The dielectric loss vs. temperature curves shown in Fig. 2 exhibit minima for both the as-sprayed and the annealed deposits at 350 °Cand 150 °Crespectively. It is believed that the network of a vitreous system is rather loose while the ion groups in a crystalline lattice are structured more rigidly [5]. Therefore, the dispersion of ion groups is more difficult in crystals. It has been reported that as-sprayed cordierite is amorphous, becoming hexagonal on annealing [3]. Consequently, both the dielectric constant and the dielectric loss display a low temperature range where the values for the as-sprayed deposit are higher than those for the annealed deposit. The strong increase in dielectric constant of the annealed deposit at high temperatures is attributed to enhanced space change polarization caused by crystal defects [6]. The concave shape of the curves of the dielectric loss vs. temperature is attributed to moisture and migration loss effects, since the deposits contain pores which absorb moisture. As a result, the conductivity is increased by the absorption of water, which vaporizes with increased temperature, in turn decreasing the conductivity and dielectric loss. At higher temperatures, migration losses dominate. The relationship between the conductivity and the dielectric properties can be written as [7] o=2irfK’K”K0 (1) where a is the electrical conductivity, f is the frequency, K’ is the dielectric constant, K” is the dielectric loss and K0 is the vacuum dielectric constant. On the basis of eqn. (1) the conductivities of the as-sprayed and annealed deposits were calculated for the temperature range where the dielectric losses
300 21
24
.20
22
~
As-sprayed
:
20
~
3/T(K)
10
8
Annealed
l~ 103/T(K)
l~
6
Fig. 3. Logarithm of the resistivity as a function of reciprocal temperature at a frequency of 10 kHz. Fig. 4. Logarithm of the resistivity as a function of reciprocal temperature at a frequency of 60 Hz and a constant voltage of 5 kV.
increase with temperature. The decrease in resistivity with temperature points to an increase in the concentration of charge carriers, such as ion groups. An activation energy (i.e. the energy required to free the charge carriers from neighboring ions and the transport of these charge carriers) can be evaluated using an Arrhenius-type equation [8]
o
I
=
E\
O~exp~j——)
(2)
where O~ is a pre-exponential constant, E is the activation energy for electrical conduction, k is Boltzmann’s constant and T is the absolute temperature. The logarithm of the resistivity is plotted vs. the reciprocal temperature in Fig. 3. Using eqns. (1) and (2), the activation energies for conduction are found to be 0.66 ±0.12 eV for the as-sprayed deposit and 0.42 ±0.05 eV for the annealed deposit. The electrical resistivity measurements are evaluated for activation energies in Fig. 4 using eqn. (2), yielding 1.48 ±0.05 eV and 1.13 ±0.03 eV for the as-sprayed and annealed deposits respectively. Both dielectric and electrical measurements show that the activation energy of the as-sprayed deposit is significantly greater than that for the annealed deposit. This can be attributed to different mobilities of the charge carriers for the different microstructures. Figure 5 is a bright field image of the as-sprayed deposit showing a homogeneous structure with an amorphous halo ring in the corresponding selected area diffraction pattern. A bright field image of the annealed deposit, shown in Fig. 6, displays a crystalline defected structure, such as the dislocation network indicated by the arrows, as well as high angle grain boundaries. A strongly diffracting grain, labelled A, has a plane normal to the [1211] direction. Fine-scale crystalline defects can be observed from a two-dimensional lattice image (Fig. 7), which was taken
301
Fig. 5. Transmission electron micrograph of the as-sprayed deposit showing a low contrast image of amorphous and halo rings in a selected area diffraction pattern (upper left).
Fig. 6. Transmission electron micrograph of the annealed deposit (1100 °C for 12 h) showing a dislocation network as indicated by the arrows and a strongly diffracting grain in the [1211] zone direction.
from an equal thickness region. Some fringes are terminated by a cluster of vacancies, as marked by V. Some fringes are distorted by dislocations, as exemplified by D. Generally, the mobility of the charge carriers is enhanced through the crystal defects at elevated temperatures, resulting in the higher electrical conductivity and the lower activation energy for the annealed deposit. The difference in the activation energies obtained by the dielectric and electrical measurements is due to the difference in frequency. The former was measured at 10 kHz and the latter at 60 Hz. The influence of frequency on the resistivity and the activation energy for conduction has also been
302
~-
‘~8151~—
Fig. 7. A (0001) two-dimensional lattice image from the annealed deposit (1100 °Cfor 12 h) showing several defects marked by the white arrows and a fringe spacing of 8.5 A corresponding to the plane spacing of (1120).
observed for a synthetic quartz crystal. The conductivities measured at high frequencies were greater than those measured at low frequencies [9]. 4. Conclusions The dielectric and electrical properties of plasma-sprayed cordierite were studied. The dielectric constant is less temperature dependent for the as-sprayed deposit than for the annealed deposit. At low temperatures, the dielectric constant of the as-sprayed deposit is higher than that of the annealed deposit, which is attributed to the difference in dispersion of ions in different crystal structures. The moisture absorbed by pores contributes to a high dielectric loss at low temperatures. The activation energies for conduction were evaluated at the different frequencies, with the activation energy for conduction being higher for the as-sprayed deposit than for the annealed deposit. This effect is attributed to increased mobility of charge carriers through the observed defects in crystalline structure at elevated temperatures. Acknowledgment The authors are grateful to V. Ben for his assistance in the resistivity measurement and to Y. Zhu for help in the high resolution transmission electron microscopy work. References 1 K. Watanabe and E. A. Giess, J. Am. Ceram. Soc., 68 (4) (1985) C-102. 2 H. S. Kanost, Interceram, 28 (1) (1967) 61.
303 3 H. G. Wang, G. S. Fischman and H. Herman, Plasma sprayed cordierite: structure and phase transformations, to be published in J. Mater. Sci., (1989). 4 L. Brown, The dielectric behavior of plasma-sprayed oxides, Ph.D. Thesis, State University of New York at Stony Brook, 1987. 5 J. M. Stevels, J. Non-C ryst. Solids, 73 (1985) 165. 6 K. V. Rao and A. Smakula, J. Appl. Phys., 36 (6) (1965) 2031. 7 W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, p. 937. 8 W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, p. 875. 9 A. De and K. V. Rao, J. Mater. Sci., 23 (1988) 661.