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Cerâmica

Print version ISSN 0366-6913On-line version ISSN 1678-4553

Cerâmica vol.44 no.287-288 São Paulo May/June/July/Aug. 1998

http://dx.doi.org/10.1590/S0366-69131998000400005 

Development of a dielectric ceramic based on diatomite-titania
Part two: dielectric properties characterization

 

(Desenvolvimento de cerâmica dielétrica baseada em diatomita-titânia
Parte dois: caracterização de propriedades dielétricas)

 

Jamilson Pinto Medeiros
Elcio Correia de Souza Tavares
Uilame Umbelino Gomes and
Wilson Acchar
Universidade Federal do Rio Grande do Norte
Departamento de Física Teórica e Experimental
Caixa Postal 1641, Lagoa Nova, Natal, RN
59072-970, Brazil

 

 

Resumo

São apresentadas as propriedades dielétricas de cerâmicas diatomita-titânia sinterizadas. Foram determinadas capacitância específica, fator de dissipação, fator de qualidade e constante dielétrica como função da temperatura de sinterização, percentagem de titânia e freqüência. O coeficiente de temperatura e a capacitância são medidos como função da freqüência. Além da corrente de fuga, a influência da tensão dc aplicada sobre a resistência de isolamento e a rigidez dielétrica foram estudadas. Os resultados mostram que as composições diatomita-titânia podem ser usadas como um dielétrico alternativo.

 

Abstract

Dielectric properties of sintered diatomite-titania ceramics are presented. Specific capacitance, dissipation factor, quality factor and dielectric constant were determined as a function of sintering temperature, titania content and frequency; the temperature coefficient of capacitance was measured as a function of frequency. Besides leakage current, the dependence of the insulation resistance and the dielectric strength on the applied dc voltage were studied. The results show that diatomite-titania compositions can be used as an alternative dielectric.

 

 

INTRODUCTION

Advanced ceramics with dielectric constants in the 20 - 150 range find large applications in the electro-electronics market. These low dielectric loss ceramics are used in high power generators at high frequencies. There is an increased demand for higher capacitance and smaller device [1]. Most of these materials are being utilized in the electronic ceramic industry for the manufacturing of electronic and electric devices such as capacitors, thermistors, VP transducers, optical window, memory element and IC substrates [2]. This work has as aim the experimental development and the production of a new kind of ceramic capacitor from to dielectric ceramic samples, obtained by sintering process of the diatomite-titania ceramic powders mixture. The sintered samples were characterized as a dielectric as a function of frequency to measurements of specific capacitance, quality factor, dielectric constant, temperature coefficient of capacitance (TCC). Measurements of leakage current, insulation resistance and dielectric strength were carried out as a function of applied voltage.

 

EXPERIMENTAL PROCEDURE

The stages referring to the ceramic powders processing, microstructural and physical characterization, pressing and sintering are described in part one of this work [3]. The block diagram of Fig. 1 shows the experimental procedure developed in this study. Dielectric measurements were performed in the following way: the sintered specimens were introduced in a metallic box with electrodes painted with conductive silver to form the capacitor plates. The capacitance (C), the dissipation (tan d) and quality factor (Q) were measured directly in a capacitance bridge (RLC), model HP 4262A as a function of the frequency of 120, 1 k and 10 kHz 25 °C. The insulation resistance (Ris) was measured using a Megohmeter MI-1050 (Megabrás), applying a fixed dc voltage of 100 V for a time interval of 2 min at room temperature. The leakage current (lf) was determined by Ohm's law. The dielectric permittivity of the material (e) was obtained indirectly from capacitance measurements. The relative dielectric constant (K) was determined from the ratio to the dielectric permittivity of the material (K = e/ e0) by absolute permittivity of the vacuum (e = 8.85 x 10-12 F.m-1). The measurements of dielectric strength were obtained applying a dc voltage between the electrodes, with the samples therein inserted at room temperature. The dielectric strength (R) was determined by ratio to the breakdown voltage (V) and the dielectric thickness (mm).

 

Figure 1: Block diagram of the followed experimental sequence.

 

RESULTS

Specific capacitance and quality factor

Fig. 2 shows results of the specific capacitance as a function of sintering temperature lying between 1200 °C/2h-1500 °C/2h to the compositions pure titania, 75, 50, 25 wt% titania and pure diatomite measured at frequency of 120 Hz. Fig. 3 shows the results of the specific capacitance as a function of weight percent of titania measured in the frequencies of 120 Hz, 1 kHz and 10 kHz. For the five compositions it was observed that the capacitance is strongly dependent of titania content, and that also have a decreasing behaviour with the increase of the frequency. Figs. 4 and 5 present the results of the quality factor (Q) as a function of the sintering temperature and of the weight percent of titania respectively, measured at the frequency of 10 kHz. For the five compositions it was observed that the quality factor shows only significant values at the frequency of 1 kHz.

 

Figure 2: Specific capacitance as a function of sintering temperature for diatomite-titania measured at 120 Hz.

 

Figure 3: Specific capacitance as a function of the percent in weight of titania.

 

Figure 4: Quality factor as a function of sintering temperature for diatomite-titania compositions measured at 10 kHz.

 

Figure 5: Quality factor as a function of sintering temperature for the diatomite-titania compositions in the frequency of the 10 kHz.

 

Relative dielectric constant

Figs. 6 and 7 show results of the dielectric constant as a function of sintering temperature in the 1200 °C _ 1500 °C range, and of titania content, respectively.

 

Figure 6: Dielectric constant as a function of sintering temperature for the diatomite-titania compositions measured at 120 Hz.

 

Figure 7: Dielectric constant as a function of titania content for diatomite-titania compositions.

 

Table I lists values predicted by Maxwell and Lichtenecker's equations for the dielectric constant of the two phases (SiO2-TiO2) systems and the experimental values found for the diatomite-titania compositions. The experimental values are within the range predicted by the equations; the Lichtenecker's relation gives the best fitting.

 

Table I: Predicted values of the dielectric constant for a two phase system (SiO2-TiO2)

 

Table II shows values of dielectric strength obtained for pure titania, 75, 50, 25 wt% titania and pure diatomite sintered at 1200 °C, 1300 °C and 1400 °C for 2h. The values found for pure titania agree with the data reported by literature [4, 12].

 

Table II: Dielectric strength values

 

Table III shows the values found to insulation resistance for pure titania, 75, 50, 25 wt% titania and pure diatomite sintered at 1200 °C to 1500 °C for 2h, under a dc voltage of 100 V. The higher values measured at the dc voltage of 100 V as a function of sintering temperature of the insulation resistance were found for pure diatomite, confirming thus its characteristic of insulator and the high values of dielectric losses (dissipation factor, loss factor) and the effects of interfacial and electronic polarization.

 

Table III: Insulation resistance and leakage current

 

Table IV shows the range of values found for the temperature coefficient of capacitance (TCC) for pure titania, 75, 50, 25 wt% titania and pure diatomite in the frequencies 120 Hz, 1 kHz and 10 kHz, respectively, in the 28 °C to 120 °C temperature range. All the compositions present negative temperature coefficients of capacitance, that is, the capacitance decreases with the increase in temperature and the results for pure titania agrees with the data reported by literature [3]. The variation of DC/Cr for all compositions are in the range of specified values of commercial capacitors.

 

Table IV: Values of TCC as a function of the frequencies found in the temperature range of 28 °C to 120 °C.

 

DISCUSSION

The pure titania specimens presented a gradual increase in capacitance values at the sintering temperature of 1250 °C. This behavior can be attributed to the growth of rutile phase. This is theoretically expected, by the fact that the ionic motion increases the internal electric field and therefore the electronic polarization rutile (TiO2), obtaining thus greater crystallinity, grain boundary growth and consequently diminution of the porosity and densification. Moreover, these specimens showed a dark-brown color, typical of oxidation, in agreement with the literature [4] and also the development of the rutile phase starting at 1200 °C [5]. At the sintering temperature of 1400 °C a sudden decrease in capacitance values occurred, indicating that at that temperature begins to occur the inhibition of grain growth and vitreous phase formation, besides the overheating phenomenon [5].

The compositions 25, 50 wt% titania pure diatomite presented a strong decay in the capacitance at the sintering temperature of 1300 °C. This is explained by liquid phase formation. On the other hand, the value of capacitance and therefore of the dielectric constant showed tendency to decrease with the increase in temperature in agreement with the literature [6]. The addition of diatomite leads to capacitance decrease. We can understand this behavior as the idealization of a dielectric structure formed by semiconductor ceramic (titania) and insulating (diatomite) grains, where each individual grain becomes a micro-capacitor. Therefore the total capacitance is the summation in parallel of the micro-capacitors of each interconnected ceramic grain, or an average of each composition, and decreases in intensity, in proportion to the increase in the quantity of diatomite, since this material has a small dielectric constant.

The addition of titania causes a global increasing in the quality factor (Q) and conversely in the dissipation factor (tan d), the inverse of the quality factor, reaching a minimum value (5%). This can be explained by the fact that titania having a semiconductive character minimizes the dissipated energy. For pure titania it was observed that the smaller values (3% to 5%) of the dissipation factor occur at 1300 °C. The greatest values of the dissipation factor (19% to 22%) are in pure diatomite, due to the increase in vitreous phase at 1300 °C. There is a correlation between the high dielectric constant and the low dissipation factor; when the dielectric constant reaches a maximum value, the dissipation factor reaches a minimum. The values presented for the pure titania and diatomite compositions are relatively high in comparison to the literature data, that are in the range of 3x10-4 for titania [7] and 1 _ 10x10-4 for silica [8]. This abrupt increase in dissipation factor is due to the high porosity in the compositions. Showing the presence of structures with low activation energies in the internal surfaces [9]. The presence of impurities in the diatomite is another reason to increase the dissipation factor, due to formation of imperfections in the crystalline structure of the material.

The dielectric constant is strongly dependent on the composition of titania and presents a decreasing behavior with the increase in frequency, as theoretically expected [6]. It decreases with the quantity of diatomite. This can be explained by the growth of the interfacial polarization, resulting from the difference in the conductivity of the present phases [6, 10]. The interfacial polarization is caused by accumulation of electric charges in the separation surfaces of the ceramic grains of diatomite ( amorphous region) and titania (crystalline region). The 50% wt titania presents an intermediate dielectric constant between silica and titania with the best results in the sintering conditions between 1200 °C/2h and 1300 °C/2h. The value measured for the dielectric constant of the rutile agrees with the range of values reported in the literature (K = 80 - 100). The maximum reached value was 90 and might be due to the porosity in titania specimens. The titania presents a uniform grain growth [11] and this homogeneous distribution, although not sufficient to eliminate porosity, nevertheless is the main factor to the decrease of the dielectric constant. The increase in the sintering temperature causes the decrease of the dielectric strength. The values found for pure diatomite are in the range cited in literature for porcelain in general [3, 9, 12]. The range of values of the insulation resistance is in agreement with the known values for low-K ceramics [13].

The better sintering results of the above mentioned composition show good densification and do not form sufficient liquid phase to deform completely the sample up to the sintering temperature of 1300 °C [12]. The titania and the diatomite do not react to form a third phase. Therefore analyzing the phase diagram of the TiO2-SiO2 system, one presents the stable crystalline shape coexistence for the mixture, the cristoballite (SiO2) more rutile (TiO2) and thus the obtained dielectric properties are part of the individual contribution of each one of these ceramic phases.

Table V compares the typical dielectric values found in the literature with the ones obtained in this work. This comparison shows that the mixture of diatomite-titania investigated satisfy the requirements to be used as dielectric in ceramic capacitors.

 

Table V: Typical values of parameters for the various kinds of components including the results obtained in this work.

 

CONCLUSIONS

Based on the results of the present study, the diatomite-titania mixtures can be used as a ceramic capacitors, with the following conclusions:

1. The composition with 50 wt % titania can be used as alternative dielectric material for the manufacture of a new type of low cost ceramic capacitor with the dielectric properties required for such applications: dielectric constant between 20 and 50; leakage current (lf) << 100 mA; dissipation factor (tan d) of 10 (10%); DC/Cr = - 5 to - 20 %,and Temperature Coefficient of Capacitance (TCC) of - 800 to - 1500 ppm/°C

2. There is no formation of a third phase between TiO2 and diatomite.

3. The sintering ideal conditions for (SIO2 - TiO2), in order to satisfy required dielectric properties for the utilization of the dielectric in ceramic capacitors, are 1200 °C - 1300 °C for 2h. For sintering temperatures higher than 1300 °C, vitreous phase are formed, favoring the degradation of the dielectric properties mainly after longer sintering times.

 

ACKNOWLEDGEMENTS

The authors thank to CNPq by finacial support.

 

REFERENCES

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[8] L. Macfarlane, "Component Technology", Plessey Co., UK (1967).         [ Links ]

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[11] K. Lichtenecker, Phys. Z. , 10 (1909) 1005.         [ Links ]

[12] J. A. Cerri, E. R. Leite and E. Longo, "Influence of titanium oxide in rupture voltage of ZnO varistors" Proc. 34th Ceramic Brazilian Congress, (1990) p.79 (In Portuguese).         [ Links ]

[13] R. C. Buchanan et al., "Ceramic Materials for Electronics: Processing, Properties and Applications", Marcel Dekker Inc., New York, (1987).         [ Links ]

[14] B. M. Oliver and J. M. Cage, "Electronic Measurements and Instrumentation", McGraw-Hill Book Company, New York, (1971).         [ Links ]

[15] W. D. Kingery, H. K. Bowen, D. R. Uhlmann "Introduction to Ceramics", Wiley, New York, (1976).         [ Links ]

[16] K. Lichtenecker and K. Rother, Phys. Z., 22 (1931) 225.         [ Links ]

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